Everybody has different opinions, but my Caterpillar owners manual says don't let it sit for too long without running it and running it so it reaches normal operating temps. Same for the generator. I usually don't have to go start mine just to start it, as we live in California and normally we use it at least every two months. But I do try and make sure it gets driven every two months even if we aren't using it. And it's not just for the engine. Everything that moves needs to move every once in a while. The rear end, the turbo, the tranny, the wheel bearings, etc.
A friend of mine was just telling me a story of looking for a used motorhome. I forget the brand he found but it was a DP with a Cat engine on a Freightliner chassis. I think it was a 2006 model and had 10,000 miles on it. The owner admitted that he would let it sit for a year or more at a time without using it. So before they did the deal my friend had him take it to the Freightliner dealer for a full checkup. They came back with a list of things wrong and an estimate of $9,500.00 in repairs. He said the mechanic told him everything was related to letting it sit. He did not get into details of what was on the list that needed fixing.