Not absolutely sure on such an older model Casita
, but anything from the 2000's and on, you will find the black water waste tank is made from molded fiberglass, just like the trailer shell itself, and is pretty strong. Is your leak emanating from the "weep hole" under the trailer next to the drain line, or is it leaking from the drain piping itself?
On my Casita
, the top of the waste tank is not only the tank cover, but is actually the mount for the toilet itself. With a little work, it can be removed for access, but it would be a last resort in my book. There's virtually nothing inside the tank that can't be reached by removal of the toilet itself.
If you still have any of the 3" black waste drain pipe still attached under the trailer, and you have a leak above the elbow, all is not lost. If you cut the black drain pipe close to the elbow on the horizontal portion, *cut it just short enough to be able to clear the frame when you unthread it off of the tank bottom fitting but leave enough to attach the flexible coupler, listed below. (Yes, the drain elbow has a female threaded upper connector and a glue-in lower, (horizontal,) connector for standard 3" ABS pipe. It's threaded into a threaded male fitting in the bottom of the tank
.) After replacing the damaged components, you can re-join the ends of the black plastic pipe with a flexible coupling with screw clamp hose clamps.
This is just informational, but this is the threaded fitting that Casita
attaches to the bottom of the waste tank. This is what the drain elbow threads onto.
If you need to replace the drain elbow
, you'll want to get a 3" "Short Turn" elbow. Do not buy a "Street Elbow" because it will hang too low!
Here's a picture of what the black tank cover, with toilet mounted to it, looks like when new: