The frame does have low R. The missing info is how long and what gauge the battery neg to frame wire is. If also 2 ft, then that leaves only 6 ft difference wire vs frame.
A complication is the inverter's chassis neg wire to frame, if used. That needs to go to a different frame spot from the neg input wire. the chassis ground is for reducing radio and TV interference.
For just 10 ft, if you use fat wire for the neg and pos inputs it would not make much difference in voltage drop running the microwave IMO, but how much is too much? Don't know.
For just wire economics for the least voltage drop you can use the frame for the neg and double up on the pos input. (Parallel both 10ft wires on the pos, and use the frame for the neg.) Now you will have way less R on the circuit.
EDIT I posted a few years ago, how I added a copper water pipe to the neg path in parallel with the frame for my converter-battery run in the 5er. Got R down to hardly any. Then I added a fat pos wire to parallel that path, and got R down to even less.
Point of no returns is when the reduced R lets the converter do its rated amps. Reducing R further is a waste because the converter is current limited.
There must be an equivalent point with the inverter. Say when you can already run the MW at 50% battery SOC long enough to do whatever just before it alarms at 11v.
No point reducing R more if you don't need to run the MW when the batteries are below 50%.
