Just had a wild thought, probably no help but.... Have you checked the air vent pipe for the grey water tank? We had a conversion done on a horse trailer adding a weekend package and the guy that did it failed to install a vent for the shower/potty combo unit and that sucker took forever to drain. Any grey water just sat there forever till enough air seeped in thru the drain to let it bubble down into the grey tank. Wasps just love those vents on the roof. When we got our new old camper our son in law told us no problem with wasps in Colorado. Ha,ha,ha! When we got it home I removed those covers and pulled out tons of wasp nests, installed some wire sink drain screens purchased at Ace Hardware and no more wasps in there now. Just something you might check. Never had a grey line clog in 46 years of camping but there's always a first. Good luck. Oh that it was as simple as cleaning those vents!