I've got about 150k on my 2001 ford van, haven't really done much to it beyond front brake issues. Probably need the plugs and belts changed. Used to tow a TT with it until I sold it and bought the motorhome. I still drive the van a few times a week. 1998 mh has about 75k and sat around for a while. Changed all 4 brakes, new tires, needs plugs, egr sensor, probably belts too. even one of the horns isn't working. Things I'd look at with high mileage would be wear items vs aging. steering, suspension, valves, piston rings, brakes, drive train, transmission. You're correct that sitting around is just as destructive as driving it sometimes.