Check out <Incaplastics.com> RV Tanks - # H136 26 gallons, or# H 86 - 27 gallons. It would fill up either the next to last bay under the floor, just forward of the bathroom, or the bay forward of that. I would like to have it as close to the axle as possible to reduce change in weight distribution as it filled. And that also reduces the moment arm on the rear of the trailer frame if I had to haul it around filled for any distance on a rough road. The Land Yacht is vastly improved now that I've added new steel aft of the wheels. It was a trailer park queen, and suffered from regular irrigation at its stern - the floor under the bathroom was compost, the insulation was still damp, the frame had holes rusted through, and the belly pan was corroded at all the rivets. Forward of the axles, all was healthy except for the belly. Even the mice.
So while replacing one sheet of plywood, 6 1/2 feet of 11 gauge 5 X 1 3/4 frames, and trying to fit a shower into the aft curb-side corner, it seems like the right time to add a gray water tank. Its going to be a little tricky getting the piping to the new tank, venting the tank, and arranging a pipe and valve for the waste.
How have any of you secured the tank to the underside and the frame or cross-members? It doesn't seem wise to weld fixed brackets to the cross-members to secure a plastic tank. Have you any experience with forming a new shower pan in the corner, and locating and installing flexible plastic or fiberglass sheets on the inner skin?
Thank you all for sharing your knowledge and experience so generously. Jim