question

I can speak to the charge controller question. 3 x 90 watts = 270 watts/12v = 22.5 amps of output. Charge controllers are sized by the output so this charge controller is a bit to small.

Since you've listed your loads as Max 15 amps you could also use the Load Controller feature of the charge controller as that will have a LVD feature.

You didn't ask but I will offer that I don't see any overcurrent protection on the wiring from either battery to your positive buss.

Assuming its the 90w Mono panels you are talking about then the 75/15 is also dangerously close to the hard limit of 75V if you are connecting those in series. Spec seems to be 24v O.C. but this can vary with temp and its an absolute do not exceed limit.

The controller would also limit your power output as previous poster says but reality is a little better as you charge over 12v normally but even at 14v, 15A is only 210w. You are really in 100/20 or above teritory here.

I am not too sure why you want 90w panels in series, why not just put a big 300w panel on? If its how they fit, if one panel ends up in shade, series panels will effect the whole string.

Seems like a very modest setup for something going in a firetruck but if you dont need more its fine. I have a bit less solar but more storage than this in a T4.

You can't just connect the starter battery to the leisure battery, at a minimum use something like the Cyrix-LI which also takes a signal from the BMS. I don't know the BMS that you are showing, perhaps that has a starter battery input, or a signal out for a cyrix?!

You could consider skipping the BMS and using one of the Vicrtron Superpack batteries instead (which include the BMS). Would mean going to a 100AH battery.

Not sure if its a simple omision but you will need a main fuse/circuit breaker to protect the battery, as close to it as possible and to make sure all the lines are protected sensibly. At the moment you have an unprotected busbar with a load of cables off it. The fusing needs work.