There are so many factors involved in a RV purchase. We bought our one and only in 2009. It is a 1998 Holiday Rambler, 32' tow behind. We went with a tow behind due to needing the space in the pickup bed for stuff. It has many items on the "required" list that met our needs. An aluminum inner frame work is best. A wooden inner frame can loosen in time and if you ever have a water leak, dry rot is common.
The very first thing is for your to decide what the primary use will be. If you are mostly going to "camp" where there is hook ups, the fresh water, grey and black water tank capacity is less critical than if you are mostly going to "dry" camp. Our trailer has 110 gallon fresh water, two 40 gallon grey and one 40 gallon black tanks. We have gone twelve days dry camping and not run out of water. You do have to alter your habits however. Turn on the shower head, wet down, shut off, soap up and then rinse. Next person in so not to waste any water waiting for it to run hot again.
Same goes for electrical. Do not skip on battery quality. We run four 100 amp solar batteries. If not already equipped, change every light bulb or light fixture to LED. Much less power usage, but remember if dry camping to shut off what is not being used or needed. Our trailer did not have gen set so I added one. Aluminum tool box on the back, 4000W Onan out of a totaled motor home with a 15 gallon fuel cell rated for in the hull boat use. I also have 345W of solar mounted on two adjustable tri-pods with 50' of cable. I can place anywhere for most sun exposure.
Another area to buy the best is tires. Everything you own behind you is riding on them. Due to the heat in the Phoenix area, I replace every four years, regardless of mileage. Wheel bearings are equally important, but many times forgotten. Industry standard is at 8,000-10,000 miles, clean, inspect and repack. If buying a used unit, definitely do this.
As stated by another, the back of the trailer has the most whip/bounce as it is going down the road. Shy away from a unit that has the kitchen in the back for obvious reasons.
Everyone talks about pulling capacity of tow vehicle, but stopping ability is even more important. The trailer brakes are secondary to the tow vehicle and are typically marginal at best. Make sure your tow vehicle has enough weight and brake capacity to stop the unit behind you.
Check out the condition of the roof. Most units have what is called a membrane roof which needs to be treated every so many years and eventually will have to be replaced. Another area that is often forgotten about. Our unit has a full aluminum roof so no need except for yearly checking caulk joints.
Storage is another consideration. Both interior and exterior places to put things is important but be mindful how quickly the weight adds up.
A living area slide out really opens up the interior. Ours has the dining table and recliner sofa.
We borrowed different units several times which gave us insight to what we liked and wanted. If not, rent a few different ones. Just like buying guns for this sport. Buy once, buy what fits you the best and have fun.
Since all controls for generator are 12V, I ran ten strand irrigation cable from it to the entertainment center. On/off, hour meter and fuel level.