This was taken from NDgent's post here on the forum. It is a great go-by list. You can do a search of thinking about buying and find his post to read it first hand.
Basically, start with the visual items –
1. Exterior
• Look for water stains and discoloration on inside walls and around ceiling anywhere vents, skylights, windows, etc. exist.
• Push and prod with your hand on the inside walls for soft spots (which could indicate a water leak and a damaged or rotten interior wall).
• Check for bubbles in the wallpaper (a sign of delimitation or water intrusion)
2. Interior cleanliness of the unit-
• Look for water stains and discoloration on inside walls and around ceiling anywhere vents, skylights, windows, etc. exist.
• Push and prod with your hand on the inside walls for soft spots (which could indicate a water leak and a damaged or rotten interior wall).
• Check for bubbles in the wallpaper (a sign of delimitation or water intrusion)
3. Test appliance functions –
• lights
• fans
• refrigerator
• air conditioner
• heater
• water pump
• range hood
• stovetop
• oven (if equipped)
• microwave (if equipped)
• stereo (if equipped)
4. Lift System-
• Does the unit raise and lower properly
• Check the lift system for hydraulic leaks (puddle under the trailer)
• Check the lift cables to see if they are in good shape and not frayed
• Check the seal between the upper and lower body halves for tears or damage
5. fill the tanks to the top and check for leaks
6. Battery
7. Solar Panel (if equipped)
