Infinite items to consider.
Congrats on your travel decision. I have full timed on and off over the last 10 years with both a motor home and a bumper trailer. Rule 1: Buy the smallest unit that will accommodate your needs. This will be much smaller than you suspect. (Two adults can co-exist in a trailer as small as my Arctic Fox 22 GQ. ) Why? Because the larger the unit the less fun it is to pull/drive down the highway, Units over 25 feet won't fit in a lot of the remote campsites, More unit means more expense, more to clean, maintain, heat, etc. and part of the fun of travel is that you leave all your junk behind. You will find that you only need half of what you bring no matter how little you take with and if you don't have something you need you will improvise. Now, what kind of unit? In my opinion, the easiest unit to use on the highway is a 5th wheel (unless you can afford a bus conversion). 5th wheels are more stable and more maneuverable than a motorhome with a tow-car or a bumper trailer. Using a motorhome without a tow car is a pain. Every time you want a quart of milk or want a RELAXING drive up a side canyon, you have to uproot your entire house. This means water, electric, leveling, cabinet doors blah blah blah. Pulling a tow car means YOU CAN'T BACK UP! Yes people will tell you that you can - just try it sometime. My bumper trailer tows beautifully but that is because I am using an extended cab 4x4 Cummins - it weighs as much as the trailer and is not easy to push around. If you chose a tow vehicle make sure it has close to or greater than a 150 inch wheelbase. An extended cab is MUCH more stable than a standard cab when using a bumper trailer. With 5th wheels it is not as critical. You will need power also. My 160 horse pulling 15,000 is just barely enough. On really steep grades like west out of Denver I am down to 3rd gear at 45. Oh yeah, don't believe any of the mileage claims people make. I get 8 to 10 towing and maybe 17 empty.
Once you decide on a type of unit, which one??? RV buyers guide is a great resource. I think they are at
www.gorv.com but I am not sure. Sometimes the library has old copies of their assessments. They look at everything - trust them not the dealers. Yes the well built units are more but the idea is to have fun and not hate your purchase. You will need a better than average unit if you are going to travel with it rather than just set it up in a park. Look at water capacity. This is the first thing you will run out of other than battery power if you decide to stay in a no hook ups camp ground. Again, trust the RV buyers guide for tips. If you can't find them, I will try to find their website.
One thing you should MOST DEFINITELY do is rent a unit and try out RV living before you buy. Take a test trip of a couple weeks with a rented unit and see if you really like full timing.
If I were to do it again, I would buy a 25 or less foot Arctic Fox or comparable 5th wheel trailer and a newer truck than my 92. I am overflowing with more information and opinions if you want them (big surprise huh?). Happy shopping.
PS: My Arctic Fox is for sale in the classifieds if you are interested. Nice unit but I need to shed some debt.