there shouldn't be a city water line going directly to the hot water heater. the city water line usually ties in after the tank water pump but before anything else and there is a check valve between them so you can't fill your tank unknowingly. so there is a good chance if there is a leak somewhere it will be weather your on city water or tank. was the leak inside the rv or on the outside? if the inside their should be a panel over the hot water heater you can pull off to inspect the lines their and see what's going on. that is the same age as my truck camper so it might not have a bypass system so you might have to access it a different way. if you can get a flashlight and see the lines in that area and then turn on the pump water it will either tell you it will work or help you figure out what's leaking