Starting an overhaul on '77 Holiday Rambler Wide World

Thanks westend! It seems to be seeping out from the bottom. And seeing how much rust is on it while looking in the outside panel I guess that shouldn't be too surprising. One thing I may have forgot to mention was that when I was doing all the testing in running water through the system I only ran cold water...is it customary for cold water to run through the hot water heater?