We have a travel trailer but a lot of it will be the same (except for the truck part)
1. Seal inspection. Trust me, you do not want a leak. Look for ceiling stains or soft spots in floor.
2. Floor Plan - make sure you are comfortable with it.
3. Features - Know what you need and what is nice to have. Outside kitchens are great but increase the price a lot.
4. Mechanical - inspect frame underneath. Beware of significant rust. Look for any cracks or broken welds. Inspect tires and brake system.
5. Structural - inspect side walls (expensive to replace) and windows (do they all open)
6. Interior - make sure Fridge is functional and temp regulates (you don't want frozen beer). Plumbing - make sure all sinks, bath and toilet can get water.
Good luck. PM me if you have any specific questions.