We, too, volunteer for a couple months at a time and now we stay put in winter. We have a diesel and have had no problems with it. We do run the generator under load for an hour/month to 'exercise' it. However, we never just start up the motorhome and let it idle which is worse for it than letting it sit.  You could take it out for a nice long drive at times but our experience is that no one does this if sitting for a period of time. I know fellow volunteers don't do this. We are very meticulous about keeping it well serviced when on the move.  .  . just our experience.  Good luck with your choice.