We dont have a quick connect but also got tired of carrying the small bottles decided I would add one. I kept running across issues folks were having (like yours) with different pressures/flows etc when researching that made me rethink. As both our bbq & shop heater have there own regulator I decided to use a tee (Camco 3 port 90ยฐ #59133) on high pressure side of the 20# bottle & I could let camper /shop external devices regulate. On camper its placed before its regulator. So far it works as intended.
More related to yours, one of the bbq issues I came across (more for back yard bbqs &bottles) is the OPD on the bottles themselves. While Ive never had this problem I've read that opening the valve on bottle quickly can result in the internal safety feature going into 'check'. This would be where you have a lot of line or open end, the valve thinks theres a leak. Its a safety thing that restricts flow. You have to shut everything down-taking pressure off the bottle and open slowly. This explains
OPD better than me. This
linkeven better, under the towables section describes but also where the valve may not be fully open and not supply full flow to system.
Ive also read where the quick couplers on some RVs have there own regulator on them, I do not know which ones. But this might have been the RV net post you mentioned,
thread I gather most of the hoses have a check valve in them-connected to a regulated port restricts flow. I dont know if this is case on Lance q/cs. Doubtful and even so I couldn't recommend clearing the check valve..
Possibly some Teflon partially blocking flow? Wonder if a propane shop can check flows/pressure on a camper?