If you are driving with an inverter with house batteries, you should be flexible to use it to your hearts content while the engine is running. Check to see that you have an battery isolator. If so, the house batteries will be on charge from the engine alternator and the inverter is just drawing from the power that the alternator is replenishing. We have a 2000 watt inverter with 4 house batteries, and my wife even runs the crock pot while going down the road. While we can't use the microwave without generator, our fridge is always drawing. So, depending on the installation, that inverter provides a great deal of convenience when disconnected from shore power.
