I agree with StirCrazy . Go shopping. Go inside. Kick off your shoes. Sit on the couch. Lay on the bed. Spend some time inside.
For sure these are not meant for full time living. Picking one unit to last you 20 years is unrealistic especially if you plan to live in it full time for 20 years. You'll have to be VERY gentle in your use to have one last you 5 years of full time. They only last 20 years if you use them rarely and store them indoors.
If you've got $40,000 to spend use it as a down payment on a house. You won't be able to save a dime for that house making payments on a $40,000 RV loan and another $40,000 vehicle loan. You'll also have a hard time qualifying for that eventual home loan with no history of renting and no permanent residence. I'm not saying don't do it, but you do have to consider the consequences of your choices.