One Way Valve with Pressure Relief

[Montana_495]

I have a 2002 3295RK. Under the sink are 2 extra water lines coming up from the basement storage area. These 2 lines are not attached to the lavatory but are hooked together with a 1 way "T" or "U" shaped valve with what looks like a pressure relief on the top.
Apparantly the valve failed while I was on a trip - got a lot of water on the bathroom floor. Then early in the morning when I got up to use the bathroom found the holding tank overflowing.
After replacing the valve with make-shift replacement parts and cleaning up - found that when the hose attahment is connected to the Holding tank flush - the replacement valve leaks. So the valve that failed appears to be tied to the holding tank system.
Can anyone assist me with an explanation of what the purpose is for this valve. Keystone told me I could simply tie them together without a 1 way valve - but in the same breath they could not tell me what valve was located in this position.I was reluctant to simply tie them together without a back flow preventer - especially now that I am aware of the possible sewer system application.

clapperjohn
2001 F-250 Turbo Diesel
2002 3295RK