Question

Truma hot water switch with red light is on but no hot water, only hot water when heat with gas.

Answer

Could be a faulty heating element or it may need re-setting.
To re-set firstly turn off the switch that controls the electric heater. Go to the heater itself and follow the electric feed wire until you come to a push fit electrical connector (normally white). Disconnect and leave for about one minute. This will allow the heater thermostat to re-set. Re-connect the wiring and turn the heater back on at the wall switch.
If the water does not warm up then you may need to replace the immersion heater itself.