Now I am worried about attending the FMCA rally in Gillette, Wyoming in July. We will go into "conservation mode" but what if we need a pump out? If I am understanding correctly, gray water should not be a problem due to all the vents. I am assuming the pump truck will create a larger vacuum than the vent can handle on the black tank? Is there some way to force the toilet flapper valve to stay open? Can't the pump truck throttle back to idle? Surely other Marathon owners have crossed this bridge. Why couldn't I stand by the toilet and continuously flush it to relieve the vacuum? Sure would appreciate other Marathon owners to give us their advice and experience.