Ford Guy wrote"When I dump the gray, it starts just fine, but then fills and holds in the drain line. If I wait it out, it takes about a minute, it then burps and flows fine at a pace that seems fine. If while the line is full, I open the empty black tank, then the flow immediately goes just fine."
My gray tank drain does the same, but the flow only stops for 10 to 20 seconds. Been that way since I bought it in 2004. I've never tried to find the problem. I agree it appears to be a venting problem. I hope you find the solution and post back here. I've thought it may be that the drain outlet allows more water out than the air admittance valves can allow into the pipes/tank and the flow stops until the air coming into the tank can catch up. I do not get odors after draining, so the temporary vacuum does not suck the water out of the p-traps.
You may not have a roof vent, as many gray water systems use an air admittance valve on the drain plumbing for each sink.
Fred & Vicki
St. Augustine, Fl.