The Heart inverter is an option I don't have on my coach.
But , and this my not be you problem at all, when tracing an electrical problem in my dash , nothing would work from the lighter socket, fuse was blown but still showed voltage at the socket
Turned out to be feed back through the wiring from the solar panel , pulled the solar fuse and volts went to zero.
The panel provided enough volts to register on meter, but of course no amps. to operate anything.
Just for fun , pull the solar fuse and see if you still get a reading.