I have a '16 Ace. When I dump I usually let the backwater go first. That seems to go in one continuous flow until empty. Then I close that valve and let the Greywater go 2nd to help rinse the hose out, but the Greywater seems to flow for a while and the gurgle like it's in a airlock situation and will then drain some more before finishing. Shouldn't the Greywater tank be vented so that there would be no hesitation when draining it?
Sounds like you may have a blockage in the vent to atmosphere (if your coach has such a vet pipe through the roof) or if not then a malfunctioning "Air Admittance Valve" which may be located (if more than one) under the sinks, or behind the washer dryer.