Welcome to the forum froggy. I doubt you will find a plumbing diagram for the rv. What make is it? Did it have holding tanks? May be one that was built for storm victems but didnt know they did the 5th wheels. Best thing to do IMO is try contacting Mfg or finding anothe same make and looking at the tank. My be hard to see because of under cover. A local tech may be able to help