I have a 2002 Winnebago Adventurer I purchase new which has a 5.5KW Onan Generator (original). It sat in storage for 18 months unused. The generator started fine when I got it out of storage and will run as long as the RV engine is running and I'm driving. If I stop, say at a rest area, shut the engine off and leave the generator running, it will stall anywhere from 5-15 minutes later. I can usually restart it but it will stall more quickly, 1-5 minutes. As soon as I start up the RV engine and get back on the road, I can start the generator and it will run until I stop again 2-3 hours later. Within 5-15 minutes it will stall. I've repeated this scenario enough times to know it is definitely related to when the RV is not running at high rpm. I have tried leaving the engine idling but that doesn't seem to be enough, I actually have to be driving (higher rpm) for the generator to not stall. I am thinking this is maybe a fuel system pressure problem? Maybe a new gas cap? I can't think of anything else and so I'm seeking help on this forum. When it is running, it runs fine, no missing or sputtering. It will sometimes sputter just as it is stalling but that is all.