A kind RVer on this form posted about the problem where you get exactly your symptoms if your outside shower knobs are not turned completely off. We had that issue (warm and then cold and the hot water tank was on, etc), and so we tried that and it was amazing! Worked like a charm. Since your RV's on/off knobs were probably turned on while servicing the shower, they may have left either the hot or cold turned on - definitely worth checking out. It saved us a service call - time and money!
2015 Providence 42M, Toad - 2002 PT Cruiser with Air Force 1 brake system. Cummins 450 ISL 9L), Allison 6-spd transmission, Freightliner chassis