Surprised no ones suggested trying to reestablish air gap? That and flushing. Couple of threads in the TCU under appliances.
The pressure/ temperature relief valves do fail or stuff gets lodged on the seat, sometimes opening, letting the handle snap close will seat. Sometimes not. But sometimes, though Ive not experienced, if no air pocket in tank, as water wont compress, pressure can cause valve to weep.
They are generic standard valves but many RVs use 1/2" threaded base, which isn't so common and can be hard to find localy at a hardware store.
EDIT: I type to slow..