Coolant leak

Sounds like the thermostat is stuck open. Some thermostats are made to fail in the open position..much better than the other way! Motorhome engines are always under such a load that only on a long downgrade are you going to notice that. In normal driving, even with it stuck wide open, the engine's doing enough work that it's going to get up to operating temperature pretty quick so I doubt you'd ever see any effect from it. All the computer knows is temperature (not that the thermostat is stuck open). In a car, the lighter duty would probably have it running too cold most of the time and the computer would probably run the motor too rich. This is no doubt happening with your motorhome going down a long hill but we all get such miserable mileage anyway you'd never know the difference. Computers are much better than old 'chokes' that would pour huge amounts of fuel in an engine (and possibly foul plugs). Computer controlled engines don't cause that problem as they control the fuel better. Unless you drive a lot of hills in the winter and are freezing in the cab, you might just want to leave things alone. Getting at the bolts to drill them out in a motorhome could be tough and time consuming. Maybe repeatedly soaking them with penetrating oil over several days might help free them. Your call... Another possibility is there is no thermostat in the engine. You could pull the hose off and look in and tell. Have you owned the rig since new? If not, that might have been done. Thermostat may have failed in the past and the mechanic told the owner that it was unlikely the engine would run cold without it (being a big motorhome). Another possibility he was in East Podunk when it happened and there was no correct thermostat available for replacement. Since you don't mention it running hot, I don't think it would have been removed for any reason other than thermostat failure, if that is the case. My personal preference is to replace with a 180 degree thermostat instead of the 195 most call for. The higher temp is mainly for emissions but I've never heard of a motorhome failing a smog check because of a lower temp 'stat. Again, your call.