I typically just use dino oil as there was a lot of controversy in boat-owner's circles when I had a boat.
The wisdom was when you store an engine for 6 months, synthetic oil might not stay up in the engine and keep a fine coating on the parts. So as good as synthetic oil is for operation, the dock talk was that it was bad for storage.
I have never seen any kind of study either way as to whether or not this is true, however, if you change your oil enough, it really should not matter if you use dino or synthetic (notwithstanding any parts coating issues).
What really does matter though is changing oil prior to storage. Sulfuric acid is produced as a by product of combustion and it ends up in the oil. The wisdom here is that if old oil is left in an engine when it is stored, then the sulfuric acid could etch the components in storage.
Again, this is dock talk, and I suppose it also depends on how long your storage season is.
I always change oil in the fall of the season, just prior to storing the RV/genny. But my storage season is 6 months. Some (boaters used to) change oil again in the spring, but I never did, as I figure the oil should still be fresh from fall storage.
The only thing that works on a RV is the owner...