Jack,
Timely post. I have the same need. I'm just about to buy and install a Victron 100/20 in my 2016 front dinette Fleet. I'll also (finally!) be installing the Victron BM at the same time.
What vintage is your fleet? I have the factory hot water system, so I think I have a different setup and less room than you, but was looking at installing it in approximately the same position (in my photo, in place of the small gray electrical box). Even with the 2 regular factory AGMs, there is no room in the battery box.
I maybe could install it where my current GoPower PWM controller/monitor is mounted, but on the back side of the cabinet fascia. I'll have to make a custom cover for hole left over from the GoPower monitor in any case.
-How easy is it to pop the cabinet fascia off? Any cautions, tips or tricks before I take it off?
-How hot does the controller actually get? Any issues with it being close to wiring, plumbing?
-Did you install the temp sensor, given that there's likely some difference in temp between the kitchen cabinet and the battery box in cold weather? Temps in my battery box are the same as outside in cold weather regardless of temp in the camper.
Many thanks for your advice and experience.