Did you know that the vent line for the waste holding tank is a small opening at the of the stanchion on the port side of the vessel? It is on my 1988 model. You can see the stanchion from the window in the head. With that said, if your holding tank is full and your pumping like crazy from the head you can literally forcing waste out the vent. Several have had pretty nasty clogs in the line. If I had to provide a guess, I'd say those marks on the port side might have to do with the vent line. Urine crystallizes. Tighten up all your hose connection at the head, especially the one going to the holding tank.
Please let us know if this was in fact the problem.
Bob
88 Mark II
