I, too, had an unknown leak from my forward grey tank on a Cougar 327RES; the one that the shower and bathroom sink drain to. My findings are that poor build engineering is not only the cause but also the reason the tank leaks.
The tanks are designed with about a 1" flange around the top edge of the tank top to which the flat cap is glued. This tank and I believe all of the tanks are installed by this flange being sandwiched between the under-floor plywood and a U-channel brace spanning from the left to right frame rails. The tank literally just "hangs" from these 1" flanges without any other support. As the tank fills, the waste water builds up at the deeper end of the tank, where the discharge fitting is located. Most grey tanks on 5er's are about 30-36 gallons, and at 8.34lbs. per gallon, this could be up to 290 lbs or more!
On my tank, a stress crack had developed on a corner of the tank at the deepest end just below the flange where the vertical side of the tank curved to meet the top flange.
A small bit of engineering could have been used to add additional support for these tanks without breaking the bank but I suppose that will never happen without many more complaints of tank leaks and having to replace tanks due to poor design.
