ok. what you could do is this ...
(this is based on the inverters sole job is to provide power to the water heater)
Replace the on-off switch on the controller with an on-off-on switch
move the existing wires from the 'on' and centre connections on the contoller to the centre and one of the 'on' connections
connect the other 'on' connections to the
BMV relays "NC" terminal (and connect the
BMV Relays "COM" to 12V)
Program the Relay with the SOC parameters - set it to say 99% and 90%.
Then if you want the heater on manually, switch to the manual ON; and if you want it on when the
battery is full, switch to the other ON.
You could do the same thing away from the
inverter by fitting an on-off-on switch to control an 240V SSR (Solid State Relay) via the
BMV and have that in line with the AC cable to the heater. This is the way I did it on my last self-build camper.
I used the CBE switch for lights and water pump, but used the left 'lights' side for the water heater, swapping out the normal switch for a 3-way on-off-on one as you see from the photo below.