That is a nice looking coach, and Marathon makes good conversions.

You can tell a lot about the care the coach has had even if the service records are not available. Some of the places I have learned people ignore are as follows:

Two year coolant changes. (look for evidence the coolant conditioning filter has been changed at the very least)

Oil level in hubs. (It should be at the line on the plastic cover. You may need to pull the rubber plug to actually see the level)

Differential oil level. (This and the previous one are serious as was seen by Lew's axle problems.)

AC filters. (An owner that is detailed in his maintenance will routinely clean the AC filters. If not, it is a fair question about what else was ignored.)

Check to make sure the odometer is recording. If it has a Detroit Diesel Pro Driver compare miles. That vintage odometer was notorious for failures. The Pro Driver is accurate.

Don't let high mileage worry you, however. Even if the actual mileage is higher it will have to be very high before it adversely impacts the coach. I would rather see high mileage indicating routine use than low mileage indicating the coach has been parked. That is what usually affects seals.

Good luck.