The valve under the bathroom sink and through the hole on the left, is a back flow preventer valve for the black tank water pressure clean out. It is unlikely this is the culprit. If it was leaking, it would drip into the basement storage compartment and be easily detectable. The under kitchen sink valves are just rubber flappers and not very effective at sealing or stopping odor. I replace ours with a household type from Lowes or Home Depot and it has totally eliminated the odor. If it is not that valve, then it may be a leak under the basement around the tank valve(s) or connection hose which means the under belly will have to be removed for inspection and repair. Good luck.
Steve & Lee
2013 Cedar Creek 36 CKTS Touring Edition
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