You are not really drawing your batteries down deeply. You have a large bank in relation to your daily needs,and your plans on charging every 4 days is detrimental to the batteries.
Batteries do not like to sit there for 4 days and slowly be discharged a little further each day. they are not going to respond well to this treatment no matter what voltages you hold them at on day 5, nor for how long you can hold them there.
Now, when you batteries are well over 80% state of charge, and you put the PM 75 amper on it, and the batteries can only take this much current for a very short while, because they are over 80% charged, and over 80% charged they cannot accept much current.
It is the lead acid battery conundrum, batteries sulfate when sitting below 80% SOC and the lower and longer they sit there , the harder to redissolve the sulfate, but they cannot recharge quickly from 80% to 100%.
Where as below 80%, they can, within reason, accept nearly everything that can be thrown at them. My single 31 will easily take 75 amps at 50% SOC. So you see the 50 to 80 strategy is actually more efficient, generator wise, than the 80 to 100. You can do a 50 to 80 quickly, You cannot do a 80 to 100 quickly. They will gas a lot in the 80 to 100 range, and use a lot of water, They will not gas nearly as much or use much water in the 50 to 80 range.
But, a 50 to 100% cycle is required, absolutely necessary, after so many 50 to 80 cycles. Your PM and a generator are not Ideal to reach 100%. The MW can take over when the PM drops to 13.6v, and shoehorn in that last 20%.
Your batteries at this point are capacity compromised to some degree. The shallower the cycle the better mentality, is not always gospel in real usage.
Since you are up north, your Max SG on those Canadian brand batteries might very easily max out in the 1.300 range. I can tell you it is very obvious on my battery, the performance when there is a .025 difference in max SG attained each day.
Try not being frugal, take that bank down to 50% overnight one night, and let the Powermax's 75 amps loose on it, and when 13.6v is triggered, take a SG reading, and start up the MW crank it upto 15v, and when the Trimetric tells you they are sucking less than X amount* of amps at 15v, take another SG reading, and dial the MW upt max voltage at 15.5x and keep taking SG measurements every so often until the SG no longer rises.
*: X is to be determined by those who already cycle 6v GC batteries successfully, not me.
Make sure there are no bubbles sticking to the hydrometer float.
The electrolyte will get hotter when equalizing, and if not compensating for temp, it can appear as if the SG has stopped rising, whereas factoring in the higher temps and the SG is indeed still rising. If you got a plastic hydrometer, give it away to an enemy, and get a glass turkey baster style.
You need to find the absolute Max SG. Don't just assume 1.275 is indicative of a full charge on any particular battery. My previous Crown batteries would get upto 1.305, my USbattery I have seen 1.290 once on a hot day. Some other times I cannot get it up past 1.285. The max SG also changes with electrolyte level, the higher the level the lower the max SG will be, the lower the level, the denser the acid and the higher the SG. Mine will not exceed 1.275 when topped up, the 1.290 reading was at the lowest level I've let the electrolyte get.
I used to have 2 group 27's for 230 Ah in the same general overnight usage as I have now with only a single 31 at 130 A/h. I'm guessing at this point I will be getting longer service life cycling this 31 battery deeper daily than the larger bank shallower daily.
But checking the 2 27's SG was a huge PITA for me and I'd not dialed in the proper ABSv or duration, or Eq'd often enough, but since they did spend so much time in the 80 to 100% range, they used a lot of water, and both sets of 27s were compromised from slight plate exposure.
Also, check you converter/ battery cable terminations for heat when the Powermax is in bulk mode providing the 50+ amps. If they are Autopart store bought 4 awg cables with pressed steel ends and a hole drilled through them, you got to get that steel off of there and get some quality ring terminals properly crimped on. I don't like your description of how the Powermax drops off early when the other charger attached kicks on at significantly higher amperage and voltage. Me suspects high resistance somewhere in this circuit.
Also It is possible the trimetric is not reading all the current if any ground cable runs directly to any battery (-) without going through the shunt. Some chassis grounds might be finding their way to ground through the engine starting battery and not being measured by the shunt.
