OP stated that he can fill the remaining half tank through the kitchen sink with a garden hose. That would eliminate this possibility.
OP...can you take a toilet plunger and "push" the water in the shower down into the tank?...does it stay in the tank, or return up into the shower back to the same level?
Jim
jwmII wrote:
Check for this. The sinks especially the kitchen could be routed to the black tank. These coach manufacturers have a way of doing things to suit themselves during construction like routing sinks to black water due to its location. Closest tank gets the job. Black of course will be located as close as possible to the toilet and many times right under it. The grey water tank will be as close as possible to shower/tub outlet and when full it'll backup into the shower/tub.
When you described the tank letting go all at once after draining a small amount indicates to me that you have a venting problem moreso than the plastic plug problem. If you can, check the grey tank for the roof vent pipe being placed to far down in the tank.