In 2015 we bought a 2000 bigfoot, ours is a truck camper.
Overall we have been lucky our rig was pretty well cared for but it showed its age.
The biggest things to look out for but in no order;
check very carefully around all windows, doors, hatches, vents, lights
and any other opening. Check to make sure the roof penetrations have been sealed.
Delamination, where the outer layer of fiberglass un-bonds from the layers below.
Condition of the holding tanks.
There can be some funky stuff growing in the fresh water tank and the black and grey tanks may have been parked partially full and then the liquid evaporated out leaving you with a mess.
Condition of all of the mechanicals and appliances.
Especially the water heater and fridge
, those two are very expensive to replace.
Often the anode rod has never been replaced and the hot water tank has never been flushed.
Check the battery
water level, if it's a flooded lead acid battery
and there is acid above the plates then it's a positive sign the owner takes care of their stuff.
Speaking about batteries
The charge converters are also suspect on older units.
I bring a good flash light
, a camera, a mirror and now a Bluetooth inspection camera.
Look under, in, behind, below and around everything. Pull out every drawer, every panel, anything that opens, slides, swings, flips or pivots should be.
Don't forget to look under the mattress.
Then check dates;
Carbon monoxide and propane
These all have a limited lifespan, some are easy to replace some more costly.
But if you have to replace them all that gets expensive very quickly.
Our camper now five years on has been great, it's still kicking along and it's still in good shape, the gel coat needs more love, and we try and wax it a couple of times a year.