2001 thor wanderer 25ft?

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Water is a RV's greatest killer. Walk ALL OVER the floor and feel for soft spots.
Look for stains in the floor, walls, and ceiling.
Get up on the roof and check the caulking for cracking / missing sections.
Crawl underneath and look at the sub-floor.

After that - check the tires for age and tread wear. You can get the age off the DOT date code. If they're over 4-5 years old they should be replaced.
Figure on pulling the wheels to check the brakes and re-pack the bearings.

some good suggestions, thanks

As with anything, make a complete and detailed inspection. Check along all interior walls, paying close attention where the walls meet the ceiling, for any signs of water intrusion. This includes inside the cabinets as well.

Look around all windows, doors, etc. for signs of leaks.

Follow your nose! If you smell a musty or mildew odor, then that means there is a leak somewhere.

Make sure all appliances work, that slides work, plumbing, electrical.

Look at the tires, the roof, and all exterior walls.

Yes I know it sounds a bit daunting, but well worth your time and you will be so much happier in the long run. Perhaps someone else can chime in and cover things that I may have left out.