Is the waste tank bad? Most by that time were plastic so short of a hole in it, it should be OK, but they are often buried under decks that would need to be cut away. I would definitely change all the associated hoses however...they have probably become porous if original.
As to the wood, if there is varnish, to do properly you'll need to sand.
Just saw your post today. I have fabricated waste tanks from 304 stainless steel when the shape required didn't match any available plastic tanks. Defender Industries offers a number of plastic tanks that have no fittings in them. You put your own fittings in where needed.
+1 on replacing all sanitary hoses. The stink stays in them after a while...
For interior varnish I am partial to Interlux's Compass Clear polyurethane. Have had good luck with it inside and outside on my Owens Concorde 27.
When I have refinished wood in my home and on the boat I am restoring, I use a clear polyurethane, either brush or spray on. If brushing, I try to put on about three coats with only enough time between to get tacky, so there are no runs. This insures that each coat bonds well with the previous one. Then sand the surface with 220 to give it "tooth" for two or three more coats. Then sand again for a smooth surface. After sanding, rub the surface with OOOO steel wool. The resulting finish is very smooth, pleasant to the touch, and can be easily repaired if damage occurs.
I prefer not to use steel wool in a marine finish as it will leave small rust spots de tongue steel fibers that are tough to ever get clean on your surface. I use 220 grit between coats and then use a 3M Scotch Bright pad for contours. Clean, dust free environment and ample dry times between coats gives you the best results! You get more satisfaction with each building coat!
Hornet, good point about the steel wool! Most of my refinishing with this process has been for house woodwork. I had not thought about the rust issue in the marine environment. I will be using your method on my additional boat wood work.