Grounding a 2nd battery for prestart heaters

[Mike Rancourt]

The simple question:
Can I have two isolated batteries grounded to the chassis that aren't connected to each other in any other way (they're power the same components at different times with a toggle selected one battery's relay or the other battery's relay)?
The context:
This is my 10 year anniversary of running WVO, and I am "celebrating" by developing a crazy plan to run a 300D as a single tank all through the usually mild PNW winter. I know, it sounds stupid (I've always been critical of single-tank conversions and only tried it briefly on one 300TD in San Diego before realizing it was inadequate), but I think I have a good plan that will use 120V AC and 12V DC to run a coolant circulation heater (AC), injector line heaters, filter sleeve heater, "vegtherm" kind of in-line heater, fuel circulation pump, and loop switch (all DC) to warm up the engine and all fuel before the engine starts (i.e. the fuel will be running through heaters up to the filter and back to the tank for 5-10 minutes or more BEFORE I crank the engine).
The problem:
I have a gigantic deep cycle battery (390 reserve power), and I want to be able to run all that DC stuff (and even the AC stuff in an emergency with my 2500 Watt inverter) off that before starting the car, then have it switch to running off the normal battery when the car starts.
A simplification:
Imagine one component--let's say a vegtherm. It's easy to use a switch to power the vegtherm either from the stock battery or the deep cycle battery (with appropriate relays of course). It would also be easy to ground the vegtherm to the chassis and connect the negative of both batteries to the chassis. However, is there any risk of damaging either battery or any other components this way? The alternative would be to run a second toggle switch for the grounds where I am selecting which ground source or the other to send to the vegtherm. Crazy, I know.
Thanks.
Mike

[Mike Rancourt]

This kind of sounds like a stupid question now that I see it in writing. Please do answer, though.
Thanks.
Mike