It is best to run the diesel about once a month to keep it in tip top condition but that is so it will run for a million miles. Not using it for three or four months is OK and thousands, maybe millions, do it every year and their diesels are still working fine. But you have to bring it up to temperature when you do run it and that means you have to drive it somewhere. Idling is not good for the engine. When you have service done techs will check your engine computer logs and if they see excessive idling, they will often tell you.

The same with a diesel generator, use for at least an hour under load, but two hours is better, about every three to six months. Run your roof air conditioners or heat pumps if you have them while you drive down the road to have a load on the generator. (Gas generators should be exercised monthly because they varnish, LP about every three months.)

Many motorhomes have the wiring to keep the engine battery charged by the inverter/charger, but some do not. Up until a couple years ago Winnebagos did not. This can be added if you find it is needed, it is a $10 part (a friend with a Winnebago Vectra had it done a few years ago and that was the parts cost.) Check your coach battery voltage after being in a campsite for a day, if under 13 volts then they are not being charged. Multimeters are your friend, you should have one even if it is a $10 one.

Edit: I should also mention that it is not just the engine that benefits when you move, the tires, brakes and axles also do better when used.  RV components work best if used.