If it’s a Fleetwood, first check is always the wet bay.  Squeeze the trigger on the water spigot in the bay, if water flows, turn off hot and cold knobs till it stops.  

if there is no flow in the wet bay, check water heater bypass valve (used for winterizing the coach). It might be in a bay, or under the sink in the bathroom closest to the heater.  Make sure it’s completely open.

if you have an Aqua-hot instant hot water system, low water pressure/flow will cause issues because not enough water flowing to activate the heater.

if none of these, other folks have had issues with the mixer valve in the water line, I’d eliminate all other possibilities before tackling this last one.