[jagizzi]

All fixable with the difficulty being defined by how much is left of the original pieces. It looks like you will have to fashion something to attach piping to the gray water tank. ABS Glue and bits of ABS shavings should allow you to make this connection again, folks will chime it here that have done it.

The rest is simply putting together the correct pieces and parts to recreate what is lost, all standard ABS parts and RV dump valves.

As far as the underside you can rivet up some aluminum or stainless sheeting with some adhesive sealant around the edges and be good to go.