Probably best to just change it out. O-rings are likely dried out and leaking. Pretty normal. Could be just something under a seat, but if it has been there long, it will have made a permanent mark, and will continue to leak after cleaning.
Happy Glamping, Norman & Elna. 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back. Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!