FullTimer goin' 5th Wheel Shoppin'

Look for a mobile rv service that's been in business for awhile. You can call and ask them if they are certified. Since you mentioned that you live modestly you don't want to buy a unit, then find floor rot. It's well worth the cost and will save you headaches in the long run. What you will need as a fulltimer does not differ from what you need on a daily basis in a house or apt. Dishes, glasses, pots & pans, towels, cleaning supplies, sheets, towels, soap, shampoo. You don't have to have a washer. There are usually laundry facilities at every rv park and/or every town.
As to a used unit look for something with good insulation, dual pane windows. They help with cold & hot weather. A used Drv would be good, but they are heavy.