TheTripp,
Welcome to the forums!
First of all, I can tell you that there is NO BETTER WAY to travel with children than in a motorhome. When we got ours (ordered new from the factory) our children were 8, 4, 3 & 11 months. We researched over 2 years looking for our "ideal RV". Note, this was also our FIRST RV, and I wanted to get it right the first time. I spent A LOT of time on this forum, and the folks here were VERY helpful in guiding us to our final purchase. (Just to put things in perspective, we started out looking at small trailers to tow behind our Honda Odyssey, and ended up with a 36' gas Class A that we use to TOW the Odyssey. Yes, we were fortunate that I could increase the budget from $15k to $100k. Hey! It's only money! LOL) We took our time, investigated all options, and ended up with the RV & floorplan that fit our lifestyle and expected use the best. Seventeen years later and we STILL like it better than just about anything else we're ever seen. Ours is the rare (for a gasser) dual-sofa layout.
As you know, travelling with youngsters in a car/pickup truck for any length of time is a challenge. I can tell you that we travelled for long days in the motorhome with the kids, and it was a breeze. Whenever we'd run into traffic, the kids did they're potty breaks and the wife would break out snacks/lunch/drinks. Heck, a few times wifey and I even switched places in stop & go traffic allowing me to hit the restroom. The great thing about it is when the traffic clears, we just keep right on cruising down the road. Most of the other cars are headed for the next exit or rest stop. We might not travel as fast as a car, but the fact we rarely had to stop, other than for gas, allowed us to keep pretty near a 50 mph average speed when on the road. Again, NO better way to travel with the kids.
You WILL need counter space to prepare food for a family. Make SURE that counter space and everything else in the rig is accessible when the slides are IN. If you're pulled over in a rest area, you generally don't want to have to put out your slides, and sometimes you can't if there isn't any space.
Bathroom(s): while more than one would be nice, we made do quite well with just one. I WOULD recommend that if there is a bathroom in the middle of the rig, make sure it is all on one side of the rig or the other. Some mid-ship baths use both sides of the aisle and if someone is in the bathroom, you have now completely isolated the front of the RV from the rear. NOT a good idea with kids. It's also not convenient when you're trying to get ready to go out and people need to get clothes, etc..
Oh, you WILL need LOTS of storage, as you already know. I've never been a fan of the bunk model Class A's, and they really were not much of a "thing" back when we were buying. But if our rig had bunks, they would take up space which would cause us to lose more than 50% of our clothes closets & drawers plus our utility closet (plumbed for a washer & dryer, but we never installed one). NO WAY could we live that way.
Heck, just in shoes, with the 6 of us, there were at least 3 pairs of shoes per person....that's 18 shoes...and the water shoes were kept in the bin underneath the rig! LOL
Oh, the Washer/Dryer setup. We thought we'd want one so we had the rig prepped for it (plumbing and electrical). But after seeing how small of a load it could handle and that we'd have to give up our utility closet, the wife decided we'd just use campground facilities. Which we have done.
Now, the longest trip we ever took when the kids were young was about 5 weeks on the road when the kids were about 11, 7, 6 & 4. Not full timing by any stretch, but we lived very comfortably during the trip. I do carry a good assortment of tools and some spare parts, but we still had sufficient storage in our rig. While travelling, we often overnight at Walmart Supercenters where we pick up supplies, spend the night and get breakfast from their bakery.
We ruled out the few bunk models that were available because we decided that "living space" was more important then "sleeping space" for the kids, and that really was true for our clan. Having those two sofas and dinette up front gave the entire family enough relaxing room to play games or watch a movie when the weather was bad, or if it was too cold out. (We fairly frequently camped well into the Fall, early Spring, and sometimes even in the Winter here in the Northeast.) Wifey and I got very efficient at the evening/morning ritual of prepping the sofas and dinette as needed. Probably 5 minutes at most. Bedding and pillows were stored directly above each sofa & dinette. As the kids got older, they helped. (OK....sometimes with them helping it actually took us LONGER...but they enjoyed "helping".) Oh, when they're young like yours, you can fit atleast 2 on the dinette, and three on a fold-out sofa. As they grew, then we needed to deploy both sofas & the dinette. But they loved it. The kids got the front of the RV at night and mommy & daddy had the back to ourselves, with some decent space in between. With the A/C's running and/or heaters or fans and the front TV on, that even allowed a bit of "privacy time" for wifey and I. (My gripe with most bunk-model class A's is the bunks are a paper-thin wall away from the adult bedroom.)
Our rig has an RV Queen bed in it and it's good for us. I'm 6' about 250 lbs., wifey is a bit over 5' and NO WAY will I put her weight on here! LOL It works find for us. I've seen King size beds in RVs and always thought that was an inefficient use of space.
You already know about outdoor storage. You should know that some rigs have basement AC's instead of roof mounted units. There might be some advantages to those, but understand they use up a fair amount of storage space. With a family, you'll need the space. Heck, you'll need a fair amount of space just for tools and things, assuming you do your own work.
For what you want to tow, I think you will be looking at a rig with a tag axle.
I'm assuming you have plans to home-school your children. If so, GREAT! We did that with our kids and it worked out VERY WELL for them. The curriculum we used was not cheap, but it was VERY rigorous and complete. Send me a PM if you want to know more. Though...it looks like you've got a few years before that's much of a concern! :-)
I think at this point I'd be tempted to advise you to look for high-quality rigs from the early 2000's or so, well kept and low mileage. In this way, most of the depreciation is gone and if in 3-5 years you decide that you need a different floorplan, then you won't lose much by selling this rig. You're undoubtedly going to learn some things as you go, and as the children get bigger, you might find a different floorplan to be preferable. For instance, a bath and a half instead of a single bath.
While we still enjoy our rig and I don't see changing it anytime soon, I think if I ever did I'd be looking for a 20-25 year old, top of the line, class A which has been gently used and nicely kept. Buying new has some advantages, but you take a hit on the depreciation if you have to sell or trade. We knew we'd be holding onto ours for a long time and with a growing family, I had to (sort of) watch the budget. But I've seen some REALLY NICE classic Prevost's and Newmars in campgrounds and I could possibly see myself being tempted in a few years after I retire.
Oh, the other thing, be sure to check for seatbelted positions. Our rig came from the factory with 12 belted positions: 3 on each sofa, 4 on the dinette, passenger & driver. In addition, I installed three across our bed so that when we travelled at night and the kids were young, we put all four of them in the bed and put the straps over them loosely. Just enough so that if I had to lock up the brakes they wouldn't go all rolling out on top of each other! LOL Anyway many rigs only have 4 or 5 belted positions. You can always add more yourself, just be aware that you might have to do that. The advantage of having more belted positions than butts to put in them is that the kids had the option to move around and change seats throughout the trip.
And along those lines, try to find a rig that has a lot of windows up front. The huge windshield of the class A and some nice glass area on the sides gives the kids the opportunity to see the sights too!
OK...I think that's most of what I wanted to tell you.
ON EDIT: Oh, one other thing...when you say "fulltime", do you mean that you don't have a physical home someplace that you can retreat too on occasion? The reason I ask is because there would be a big difference for me between packing the rig for being out on the road knowing I can come back home if needed, and packing it with everything I own that I want to keep with me. I bring a cross-section of tools with me in the rig when we travel, but no way could I bring everything I wanted to have. I guess it depends upon individual preference, but I've amassed quite a selection of tools over my life (60 yrs. old now) and I'm not ready to part with any of them now. IF wifey and I become full-timers when I retired, I plan on keeping the house. I'm sure at least one of the kids will be interested in living in it while we're away. But to pare down EVERYTHING...THAT would be real tough for us.
Good Luck in your search. Take your time and please don't hesitate to ask us questions!
~Rick