The Truma knob/button LCD displays are finicky. Mine was sticky, which made it unreliable.
It sounds like you have an issue with the heating elements. Does it work on propane?
Are you getting any error codes?
FWIW - I've had good luck simply contacting Truma USA via either phone or the web form. In both of the cases that I opened with them, they were very responsive, even spending hours on the phone helping me troubleshoot.
https://www.truma.com/us/
If you are OK with driving to N. Indiana, Truma will service the unit in their shop.
--Mike