My concerns do not have to add up. If the manufacturer recommends a dedicated fuel source, then I heed their advice rather than second guess their reasons. I'm also sure that you know the diesel propulsion engine requires more than just the small amount of fuel it burns. Most of the fuel that is pumped to the engine is used to cool and lubricate the injector system after which it is returned to the tank. I'm sure that when Hunter designed a boat's fuel system, they did not build in some additional capacity in the unlikely chance that some owner would be foolish enough to tee in another device.
Regarding the economical merits of 120 volts AC electricity versus diesel fuel to heat the boat while on the hook, please consider other factors. In our case it would require that we operate our somewhat noisy 8KW Genset that consumes about .75 gallons per hour to produce the electricity needed to operate some electric heater big enough to keep the boat warm. Or a very quiet forced air furnace that consumes about a pint of fuel per hour. I'll take the furnace.
At the slip where we have access to shore power I operate the boat's heat pumps, which are far more efficient at producing heat than some electrical space heater. On the other hand, I do operate a small electrical space heater set on low to help keep the boat dry and mold free while unattended, but nowhere near a comfortable temperature.
Terry