Ours did the same thing (1982 Pace Arrow) ours was clean water at least. It was the vacum valve, I think it was like $35.00, might as well get the flange gasket too. Its in the rear of the toliet so you will need to take it off the floor. 2 Bolts, 1 inside the top there is a access cap, pop that off and you will see it, the other you can get from the rear. I was glad as we had just bought it and really wanted to have it really cleaned good. I used Bleach White (yes for tires) and Dawn and the garden hose in the back yard and looks like new. You might find it easier to remove the toliet seat too. If you google Therford(sp) and look in there search "Vacum Valve" it will show you the part and part number. CW has them. We order ours from a RV dealer.
Total job with good cleaning was about 1-1/2 hours, tops. Let me know if you have a problem I have the Brochure. You can also find a how too Video
Good Luck, Tim
1982 Pace Arrow 30ft.
KarKaddy SS, Toad:99 Concours
Lilly and Rubie Our 4 legged Kids & Tim & Joe