OK...so the OP really likes his coffee and there is no reason that he can't have it in a MBS as long as the engine is running. All of us with residential reefers do the same. He can do it this way:
1. A 2000 watt inverter like a Xantrex ProWatt will draw a lot of power when running, like more than 150 amps at full power, and the MBS dual battery setup will only be able to supply that full load for 45 minutes or so. I do wince about running a hair dryer off of a couple of poor 12 volt batteries, but it can be done. My assumption it that for more than say 500 or 750 watts, the engine and alternator will be running and supplementing the battery power.
2. The reason that I recommend the Xantrex (besides the fact that I have two of them) is that if you install their remote on/off switch, it has a line that goes to the ignition switch which will automatically prevent the inverter from running when the engine is off. (there is a switch to defeat this however.)
3. A Xantrex automatic transfer switch (nice little box) is installed between the inverter and the outlet circuit that it feeds. This will automatically switch to shore power, dropping the inverter when 120 volts is available automatically. The closer to the breaker feed that this is installed will enable as many outlets for "automatic" AC power as possible.
4. Very heavy gauge wire from the batteries to the inverter and as short as possible. In the MBS under the sink would be really close and installing the remote on/off switch on the bulkhead by the stairs would be easily reached. The transfer switch goes inside the same cabinet and intercepts the feed from the breaker to the outlets that are powered.
In short, this ain't rocket science. I love having all of the outlets in my coach (with the exception of the kitchen outlets) always powered when running down the road, and love to watch TV without resorting to firing up a 5000 watt generator!
The remote on/off switch:
The automatic transfer switch: