As spring time approaches, one of the most common asked questions that new recreational vehicle owners, as well as seasoned veterans who sometimes forget ask, is "why don't I have hot water"...or "why do I only have a little hot water before it turns cold?"
This occurrence is usually caused by the bypass/crossover valves being set incorrect for normal use. The bypass valves are closed during the winterization process, so that the water heater is bypassed and can be drained of any water that could freeze. The crossover valve (which connects the cold water line to the hot water line) is then opened which allows antifreeze to be placed into both the cold and hot water lines via the RV's water pump.
What happens during springtime, is that members who weren't shown this when they bought their RV, or just forget the proper dewinterization procedures, then will not have ANY hot water coming out of their hot water taps if the bypass valve is still closed (it will only be cold water from the crossover), or will have the cold water still mixing into the hot water line if the crossover valve is still incorrect....which results in only having a very short period of hot water coming from the tank until the cold water is drawn in from the crossover line, causing the water to turn cold quickly.
Many times these bypass/crossover valves are going to be located behind the water heater itself. You may have to remove a false panel, cabinet drawer, bed bottom, etc etc to gain access to these valves. You may have only one, or two, but possibly three valves to adjust for normal usage. The easiest way is to locate your water heater on the outside of the RV and then go inside and figure out how to access the rear of it. Some RV's will have a plumbing station (pic below) that has the bypass/crossover valves there.
Once you do gain access, you want to make sure that the cold water inlet line valve (if equipped) is open going in to the water heater. The hot water outlet valve (if equipped) is open coming out of the water heater....but most importantly, the crossover valve is fully CLOSED between the cold water line and hot water line.
Since the crossover valve and the lines it's on are usually running vertical, as opposed to the hot and cold water lines going to the water heater which are horizontal....it's real easy to mistake the position of the crossover valve. You may think you closed it but it's actually open. The handle must run perpendicular to the line, for it to be closed on a three valve set like below.
Here is a schematic that may help:
Plumbing Station/wet bay (styles may vary)