Elisa, I have experienced a similar problem. The gray water valve was stuck
open. Some progress has been made by repeatedly putting septic tank enzyme
powder in the holding tanks, after each dump and with some water when the
trailer is not in use, for an extended soak (enough water to immerse the
blade valve). The enzyme substance can be purchased at Walmart or most
large grocery stores for 4 or 5 bucks and will do for a number of
applications. The cheaper brands seems to work as well as the expensive
ones for this purpose. I have not achieved 100% sucess, but both valves now
close and open and the amount of leakage is decreasing and sometimes there
is no leakage. I do envy you having the outlet at the rear bumper and I
plan to modify mine to be able to attach the sewer hose at the rear bumper.
My dump outlet and valve handles are under the trailer on the street side
and not convenient to get to for opening and closing. The ensymes may not
solve the problem, but I recommend you give it a try. Ned Digh