There have been a few threads on this. I believe the concensus is that it only gains miles when it is being driven and the ignition is on. They have actually driven it 77k miles. It could have 200k on the other components/parts. How many miles are on the motorhome? If they have had the jeep since the motorhome was new, those miles + 77k are how many are on the jeep's tires, suspension, steering components, ...etc.
The owner's manual has a section detailing the 'tow-behind' procedure. In theory, the tires and wheels get a lot of action while the rest of the Jeep comes along for the ride.
I bought a 'tow-behind' myself a couple of months ago. It is an '04 and the odometer registered 5K - so it doesn't seem like the miles add up the vehicle being towed. That particular motor home toured back and forth across the country for 4 - 5 years.
The Jeep had two 2" towing channels mounted to the front frame so the two vehicles were locked together. The bouncing and braking action of the motor home would directly impact the front portion of the Jeep since the Jeep's front axle was slave to the movement of the rear of the motor home. When I got the Jeep, the front tires were cupped radically while the rear tires were not cupped and looked to be in good shape.
I immediately replaced all the rubber and moved up a couple of sizes for appearance sake. Since the initial ride home from the seller was pretty shaky. I stopped to buy new tires even before I got home. Talk about DW! Then during the next few days, I set about removing the front towing hardware and cleaning up the electrical which had been relocated.