Instead of running a wire under the bathroom floor to go from the sink to the rear battery compartment, which is a major PITA (I've done it), you could consider using a Victron Bluetooth shunt. I have both a wired shunt (with a Link 10 battery monitor) and a Bluetooth shunt (Victron's "Smart Shunt") between my 2 TMs, and they're both great. The Bluetooth one wasn't around when I put the Link 10 in.
https://www.amazon.com/gp/product/B0856PHNLX
The only disadvantage of the Bluetooth shunt is that there is no permanently mounted display, since it connects to an app on your phone. Victron may make a Bluetooth monitor though for that purchase perhaps, or you could repurpose an old cell phone too.
Dave