I am thinking of using a little heater for my TAB 400 BD camper. It draws 6.5amps or 750 watts. D

The short answer is there should not be a problem plugging in a 750 watt heater 6.5 amp, when you are on shore power. If you propose doing this from battery power using an inverter, the calculation changes.

But since the T@B 400 has both the Alde or the Coolcat that can provide heat, the question is what you are trying to achieve with the small heater.