The DSI; ( Direct Spark Ignition ) light is to say that the propane operation is in the process of lighting up and goes out when the propane stays lit. Normal operation, for the propane function. If the DSI light stayed on , that would be an indication of a no start condition.
My heater doesn't have a FTL light so I can't even say what the acronym means.
However you say the MAIN breaker trips , do you mean all the 110 volt power, in the RV; goes off
or is it just the dedicated breaker for the water heater that trips ?
The questionable water quality in many parks can corrode the 110 heating element inside the tank causing a short , and tripping the breaker. I have to replace my element every season , but I live in the RV 5 months at a time.
If your problem is that , just the water heater C/B trips then the element is probably shot , approx $15, instructions for replacement should be in your owners manual.