Most RV water heaters have what's called a Back Flow Preventer, or Check-valve ($10.00), where the cold water line connects to the hot water tank to refill it. This simple "one way" valve consists of a small ball and a spring, which allows the cold water in to refill the tank, but prevents the hot water from escaping back into the cold water line. If the ball gets gummed up or stuck, no cold water can enter the tank, thus no pressure and no hot water from the tank. "Some" RVs also have a similar check-valve connected to the tank outlet, to prevent anti-freeze from getting into the tank when the RV is winterized. This valve can also get stuck, preventing the hot water from leaving the tank. The inlet valve, however, is most likely the culprit.