TO follow 2Oldman's comment, the water heater is pretty noisy when it's on. I suggest you turn it on, go outside the coach to where the water heater is located, to make sure it's one or not.
If it is on, let it heat for a while, then check to se if you have hot water come out of the pressure relief valve at the hot water heater. If there is, then this is a pluming issue.
If no hot water, this could be the heater's igniter, or fuseable link. Mine went out last years, bought one, installed in about five minutes.
Good luck, hope you find it.