The diesel gen is self priming...hold the button until it starts...so improper priming isn't an issue. The delay is mostly waiting on glow plugs to warm up. My guess also is low oil or insufficient fuel delivery. It takes a few seconds for the engine to reach proper oil pressure...if it doesn't, it will shut down. Call Cummins/Onan...they are great at helping over the phone....of course, after you check the oil level.
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie