I hesitate to post this but I suppose it will be worth it if it prevents someone else from running into a similar situation
. So, short answer, it takes a honey wagon and a very specific set of circumstances. We just got back from a trip to a NASCAR race and then Disney’s Fort Wilderness. While at the race, and knowing I’d have full hookups at Disney, I chose to use the honey wagon to dump. Everything went smoothly and we proceeded with the trip. I like to fill the holding tanks before dumping so at Disney, I did not hook the sewer up until just before leaving the campground at the end of our trip and this is where everything comes together. I have two 3 inch dump tubes with Valtera valves at the end of each. I dumped the rear first which went smoothly(didn’t use the honey wagon for that at the race), sealed things up and moved to the forward grey tanks and black tank. I went to open the Valtera valve first and there was some resistance which released suddenly and the sewer hose completely collapsed.
I’m sure some can guess what the cause was. The gentleman operating the honey wagon asked me to close the grey and black valves before disconnecting and then closed the Valtera valve apparently trapping a vacuum in the forward tube. Having dumped the rear tank first at the end of the trip and moving the sewer hose, there was liquid sealing the end of the hose. Had I opened the black or grey tank dump handles first, the vaccuum would have been released through the tank vents. However, with seals at both ends(tank valves and hose), the opening of the Valtera valve released vacuum into the hose which clearly was the weak point collapsing it.
I’m sure the planets will never align like this again but its now something else to remember.