Welcome to the forum.
I would unplug all the plugs going to the circuit board and make sure all the contacts are clean. A pencil eraser works good. Them plug them all back in several times to clean the contacts you can’t reach. Do this to any other plugs also.
Does it work on electric only?
Please let us know how you make out.
Must have good 12 volts to the control circuit for the WH to operate.
If you can smell gas and hear a clicking sound at start up, it is an ignition problem which is usually the spark/sensor either being bad or misaligned.
If no gas is present, that indicates the valve is not opening for ignition which could be a lack of 12 volts to the control circuit or possibly a bad thermostat or a tripped high limit. The high limit probably requires a manual reset if it has tripped.
Resetting all plugs and connectors is always helpful and google the WH to download the service manul.
You can download a copy of the service manual from Bryant RV Services. You should have 12V power to the t-state with the 2 brown wires when your water heater is on in either mode and that thermostat should be closed (0 Ω between the two pins with leads removed). If it is open no heating mode will work as is true if you have no 12V power.