Our Class C stays outside uncovered and hooked to the electric service 24/7.
I feel that a cover, not a garage, while they have advantages they have the disadvantages of wind movement and lack of ventilation.
I exercise the gen for 30-45 min per month and start the main motor more often.
The motor is started at least that often and run until at least until the temp gauge stays in the normal range. Prolonged periods without starting can lead to dry starts as the oil has rained back into the pan. Even though is only a second or two, it takes it's toll.
The transmission is also placed in it various positions to direct fluid through the valves, etc.
I also apply the brakes several times to exercise the calipers and check the electrical systems and heat A/C, etc. while I am there.
I feel it all a part of normal maintenance.