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I am not sure if Fleetwood has plumbing diagrams going back that far. I am enclosing the link to their Customer Service Number. They are part of the Allied group now. http://www.alliedrecreationgroup.com/service/manuals.asp
You may be able to see the problem from the service bay and it is probably a pipe issue at the tank.......but, might be along the line so look at all the plumbing you can first. Please let me know if they have some diagrams. Get back to me if you need more on this as all follow ups are included in our fee.
All the plumbing can be accessed from one of the service bays and some have to have a panel removed in order to see it. Can be kind of a challenge as they build these things as though nothing would ever break on them! I have had to cut holes in some of the walls sometimes in order to replace broken pipes. It can be a tight squeeze, so you may need a small person to get into some of these spaces!