Welcome aboard.
When looking for a used RV, besides the afore mentioned water damage and other obvious problems, like tires and appliances that do not work: Remember that Floor plan, Tow Vehicle and Cost all have to match.
Floor plan is something you like as is. Moving things after you purchase can be difficult because behind or under everything is something else.
Tow Vehicle generally run out of max cargo capacity before towing capacity. You can pull a lot but you cannot haul a lot. There is usually a sticker telling how much cargo you can carry. Total up every thing you are going to put in the tow vehicle then add the loaded trailer tongue weight.
Cost, as with any vehicle RV depreciate rather fast. This makes the used market attractive.
Since the DW and I did not know exactly what we wanted; we rented a similar sized unit first.