It sounds like when you turn on the hot water knob you are getting cold water? Have you tried the hot water heater on both gas and electric?

On gas can you hear the flame? It should be fairly loud from the outside if it is working.

If it's not working on electric have you checked the circuit breaker? You may need to replace the heating element. If by accident the heater was turned on without water in the tank that will kill the heating element. They can also just fail over time.

If you aren't getting any results on either gas or 120v electric you may have a bad t-stat . Here is a video on repairing that:

https://www.youtube.com/watch?v=uYzqP2BFrMk

I can hear it fire up then it heats but shuts off the electric I can turn on and hear the element heating again no hot water

By chance do you have the hot water bypass on. It the valve when you winterize your unit?

I replaced the thermostat and it fired up got hot then quit and light that says pilot light came on water is going through ok even the electric heater you can hear that kick on too but no hot water the fuse is good on the water heater, only thing left is brain box if there is a bypass where in the hell is it manual does not show hell of a lot I thought air in the gas line but the stove has a real nice blue flame so any more suggestions? My next step is to call RV technician We bought used and they had un- winterized. We do not know where the bypass valve would be located.

The bypass is usually right behind the water heater. Google water heater bypass location in rv. This will give you an idea where it may be.

The bypass valves are located on the inside of the rv. Find the back of the water heater. You will see 2 pipes enter the water heater. One on the top and one on the bottom. Each one will have a valve. They deliver and discharge cold water in and hot water out. The bypass will be a tube that runs between those hot and cold lines. It also has a valve. You want the bypass valve closed and the other two open for normal operation. For winterizing it is the opposite. (like picture below) If the tank is in the winterization configuration with hot and cold closed and bypass open and you have been running the tank on electric you may have destroyed the heating element.

Here is a typical set up. Note the 3 valves. One on the top, one on the bottom and the bypass valve between them. Top and bottom open, bypass closed for normal use. These valves are open when they handles run parallel to the tubing. In the picture below it is in winterization mode. All three handles need to be turned for normal use.

There is water in the tank I turn on hot water it comes through I have found the valve that stops the water going into the hot water tank that is open the water heater kicks on runs then shuts off and the light for pilot light is on telling me it failed

I found out it's the bypass was on now there hot water!!! Thanks guys the repair man show me few more things to!!! Thank you all!!