Used, Used, Used.
Three to five years old. Not only the RV rig, but the truck if you go that route. Since you are looking at only a few years - don't invest that much money in something new.
If you buy new and sell in five years, you might get 35-45% of your money back. If you buy used and take care of it, you might get 65-75% of your money back.
RVs have very poor resale compared to purchase price for new units.
No doubt a 5er floor plan provides much better living space. If you get a spacious 5er, you will need a 350/3500 one ton diesel pickup, preferably with dual rear wheels. And I'm seeing more and more people with F-450 or F-550 trucks. That is going to be a $65-75 thousand new vehicle. Add a similar price for a big 5er, and you are approaching $150,000. Not worth it for a five year plan.
Do not skimp on a tow vehicle truck. Get the biggest you can possibly afford. Few things in RVing are more trouble than having too much trailer behind too little truck. It's not just weight. The bigger the rig, the bigger the sail area that can push the rig around while traveling down the road.
Now about 5th wheel units. I've seen eight couples I know leave fifth wheels in the past four years. All because of the steep stairs in 5ers. Two had major falls, the others gave up the rig in favor of a flat floor due to hip/ knee problems.
Three went to Class C units. Two went to travel trailers, One went to a Gas Class A. The other two gave up RVing.
Another important consideration is setup/ tear down time. Class C and Class A units generally are faster and less 'work'.
You definitely want a unit with an automatic leveling system - preferably hydraulic, but electric might be the only option in a certain model. Those are okay, just not as powerful.
We have a TT with opposing slides in the main living area, and a slide out bed. We love having the Sleep Number bed, and the great space in the living/ dining area.
But just like the similar 5th wheels - we have to do a Chinese puzzle box routine of moving stuff around to get ready to fold in the slides. Make sure any salesman shows you the rig with the slides out, and with the slides in. Visualize how much work it is going to be to get the rig ready to fold in the slides.
You can't just slide them in. You are going to have to ensure everything is braced, tied down, placed so it doesn't become a wrecking ball as the rig rides down the road.
At campgrounds without sewer connections, we frequently see people with Class A or Class C units pull in their slides and drive to the dump station. Return to their site after they have dumped the tanks. Over the past three years, I've seen one TT do that. Folks with TT and 5ers usually have to have a sewer tote to dump part of the black and gray water into the tank, take it to the dump, repeat.
I have 70 gallons of black and gray tank. I have a 27 gallon sewer tote - the biggest I can comfortably move on the ground alone. Three trips to the dump station is my normal routine every 7-8 days at campgrounds without sewer connections. I have a Thetford - take my advice - buy a Barker Blue Boy.
I most strongly recommend a 50amp rig with two air conditioners. We are currently sitting on Lake Lavon at East Fork - one AC is NOT going to keep the temp inside the rig below 90 today.
On I-35 just north of the Denton Camping World is a consignment lot with several Class A and Class C units for sale - browse through there in a couple weeks - after the daytime temps drop out of the high 90s.