A couple of things come to mind that don't necessarily correlate with age or mileage. Go inside your preferred unit and examine the areas around the windows, vents, AC etc. and look for evidence of water penetration. Take some deep breaths and try to determine if there is any odor of mildew. (Be cautious if the owner has liberally spritzed the unit with air freshener prior to your arrival). Ask for maintenance records for the truck, and seek evidence that the generator was run periodically (bi-monthly or monthly). Check tire dates and the sidewalls for cracking. Get on the roof and check the seams and the areas around vents, AC, TV, etc. Are there obvious cracks that need to be repaired? What condition are the truck and house batteries? What is the OCCC of the unit and is it large enough to meet the needs of you and your family?
Best wishes for finding your unit!
Bob