1) I would check the tires - Goodyear LT in the 16" or 17.5" are a better choice and the spare.
2) The wheel seals - look inside the rims for signs of grease.
3) The slide out seals, both inside and outside, above and below. Plus those "D" seals.
4) as mentioned above - The Roof material and all seals, seams, roof vents, skylight, a/c shrouds + possible faded fiberglass front and rear caps.
5) Appliances - Refrigerator, stove, microwave, dish washer (if installed) heater, A/C, Generator (if installed), Battery and Inverter/Converter.
6) Look for possible water leaks inside, under sinks, storage bay, carpet near slides, etc...
7) Do they have any repair/maintenance receipts (would be nice) and the owners manuals.
And this is just my short list - don't forget exterior paint, graphics, under belly, carpet, furniture, cabinets, ceiling, lights and switches, electronics (TV, Radio, Speakers, Antenna, TV Booster, etc.)
Maybe a video camera to use during your walkthrough - cause most will only remember 50% of what was said, if that much...
This is just my list, hope it helps!! Good luck and hope to see you back here as the "Owner".