What are you going to tow with? That will help to determine what size/weight you can tow. The heavier the better. That usually mean more insulation and thicker walls.
My wife are going your route but in a slower time frame. We already have a 5'er but nit one we would take on 2-3 month long trips. We are on our 3rd trailer and pretty much know what we would want for longer term travel.
For us it's a rear living unit with a sofa/recliners facing the tv. We have had enough of the rear living units with the tv at a 90 degree angle.
Lots of counter space. We cook 90% of the time and counter space is a must. Also a pantry. Not having a pantry would mean compromising other cabinets.
A bathroom large enough to be able to shower and get out and dry off without contorting yourself. No more corner showers either. A 48"x30" shower at minimum. Two doors into the bathroom also. With one off the bedroom and the other in the hallway. Look at how the doors open into the bathroom. Some are back a$$wards. If you're in the bedroom and need to use the bathroom some have the door opening from the stairs, which means you need to step down a stair to get in from the bathroom.
Bed slide only. No closet slides. That gives you more room in the bedroom. Also make sure there is enough room between the bed and the sliding closet doors.
Can't have enough storage. Look for as much as you can get throughout the 5'er.
As far a frames go unless you get a Arctic Fox or one of the high end units they all have Lippert.
On insulation, you need as much as possible. But be aware that there is a lot of mumbo jumbo when it comes to what a true 4 seasons 5'er really is. The use of Astrofoil is questionable at most. Some of the R values seem to be overrated. But there's not much you can do about it. Just look for the highest R values you can find. Another issue is how the 5'er is constructed in terms of air leaks. Unless you start taking off access panels or removing drawers you won't really see how they sealed all the cutouts for exterior items.
We found that we will need at least a 32-34' min to get what we want. Can't park much one that is much longer at our house so that's our limit. If it weren't and issue then we would definitely look in the 35-36' range. We don't need a 38-40' 5'er and would be a PITA to park at some of our favorite sites. Let alone traveling to unknown CG's round the country. We don't like the RV park scene at all. Way too close and usually crowded. So in our travels we stay at forest service or state park CG's.
My advive would eb to go to some RV shows and spend some time in one. Sit in all the chairs. What looks comfy isn't always comfy. Check out the whole unit by going thru the motions.
And don't let the sales guy try and sell you anything. It's rare when you can find one that's interested in what you want and not just trying to make a sale.
Check the payload ratings on your truck to make sure you're within the ratings. You don't want to get a 5'er then find out the truck is maxed out. If you haven't got the truck yet, then get the 5'er 1st and then find the truck that works. It's a lot easier to find a floor plan you like than it is to compromise cause your bound by your trucks limitations.