Usually very close to the water heater. My Cherokee has two valves, one on the cold inlet and one on hot outlet of water heater. There is a loop of hose from one valve to the other.
It's going to depend on the model of your TT or 5er or MH.
Look where it is out side and measure from,the door then do the same inside and see where that is. Door, cabinet or false panel will be where you will find the valves. Mine are deep under the kitchen sink at floor level. Hope this helps.
2012 SunSeeker 3100SS Toad-1962 Futura Average 100 + days camping