Same basic arrangement as the C28 with the aft end of the keel glassed-in.
Must admit it has always been a keel tank to most of us, never heard of a tail tank before, but it is descriptive enough to indicate what you mean.
Lots of the boats (28 and 29) have cut a 100mm dia inspection hatch
under the engine
, but not a practical solution with the engine in-place still.
I assume you are also looking at the Forum on the Compass site on Google
Groups. We are quite active there , especially for the 28 and 29s.
Most likely cause is a blockage in the fill pipe. I assume you have the standard arrangement, with a 1 inch flex hose from the top of the tank, straight up to the bridge deck
mounted filler. There were a lot of boats home completed, so the inlet pipe may even be taken down to low level in the tank. I wonder if the pipe itself is deformed, but then you say you have removed it already.
Try removing the outlet hose, which has a fixed dip pipe inside the tank, dropping close to the bottom. Then use bellows from an inflatable
boat or similar to push air into the tank. Cover your eyes but keep close and listen as well as watch. Weird gurgling and other strange sounds indicate an excess of sludge. Boats of this age tend to accumulate such.