So I wasted a bit of internet time looking up "idling the diesel". The VAST majority of guys who idle theirs do it "because it's what I've always done with diesels since 1922" or similar.
Looking up the specific owner guides when available leads to statements not a lot different from those posted here a page or so back: 3-5 minutes cooldown period after a long or heavy run.
To me that means idle as you wander into the RV park, check in, check your site, and back it in.
I've no idea where the 30 minute idling idea comes from.