Varnish is the real culprit. Replaced a few carburetors on hot rods and powered garden equipment because of it. Varnish is difficult to wash away however I agree with the sea foam idea. I have Sta-Bil in all my
fuel cans and start my portable generators once a month and put under a load . You need to remember also the air conditioning system needs to run on a regular basis ,even for a short time, to lubricate the seals.
This pertains to the coach as well as the chassis. When air conditioning loses freon that is usually why. I would add sea-foam and drive the coach down the road for a half hour or so. It will do your transmission
some good also. Don't forget to run your generator if so equipped.
As far as the oil, if it is fairly clean running the engine up to temperature and then some will remove any condensation from the crankcase. Also check the water
in the radiator, rust will eat your engine alive. When running the engine turn on the heater to circulate the water through. Be sure the anti-freeze is good. I
always add distilled water to my radiator, battery and windshield washer bottle. It's inexpensive and it helps .