Well just our luck, we get a good looking 2005 dodge ram 3500 with only 129000 miles on it to pull our 5th wheel and my wife gets sick. we have been looking for a good truck when we found this one. ...
Sitting is better than idling - nothing good happens to an idling engine.
Each thermal cycle does some damage.
Moisture accumulates in engines and fuel tanks.
Summary: Take it out and run it hard every so often. Once a month is more than enough. If you miss a month, don't worry. Keep the fuel tanks full to minimize "breathing". Diesel fuel takes a very long time to get old. Keep a charger on the battery(s).