I agree, it depends on mechanical condition, not age. The chassis was designed for trips like this, taken at a leisurely pace. At 25 years with only occasional use, you need to be sure of condition of tires, wheels, brakes, cooling system (especially hoses, including heater hoses), engine, transmission and axle fliuids, wheel bearings, drive belts, fan clutch. If you want to stay cool as you move west (Midwest and Great Plains get very hot) make sure airconditioning is in top condition.
A rare long trip on an old vehicle requires that you do the preventive maintenance you've deferred, which might include replace of old, unworn, tires. My summer trip breakdowns have involved replacing tires, cooling hoses, fan belts, including right after a dealer servicing and so-called inspection. Tires, it was "these are good for one more trip, I'll replace them this fall."
Only other advice is to make sure you understand basics of mountain driving, controlling speed to minimize braking. Most of the routes in and out of Yellowstone involve mountain highways, you can't get there using only the Interstates.
Tom Test
Itasca Spirit 29B