If the 75 amp PM quickly gets the batteries upto 14.6v, then the batteries were in the 85%+ range to begin with. They are not going to accept a lot of amps at this point, They are not going to recharge quickly from this point no matter how big of a charger or how high it pushes the voltage. 80 to 100% can and will take longer than 50 to 80 as batteries at 50% can take everything you can throw at them and amps tapering only being in the 80% region if the cabling is adequate and the batteries are still somewhat healthy.
Your batteries could be seriously compromised at this point from chronic undercharging and hardened sulfation, and all the time in the world at 16v is not going to restore them to their New A/H capacity, but you might as well try once or twice and use that as the new 'good as it gets' baseline.
Are you driving this vehicle to the work site? Are you taking advantage of the alternator? It can be a very effective bulk charger if thick wires exist between alternator and battery bank. I think the physical agitation of driving, while also charging at a high rate, helps stir things up, helps max out the SG.
Any and all charging sources as often as possible when cycling them. Power Pedestal users need not contribute to boondocker discussions.
The Op needs solar, a good amount.
I think I'd modify the PM to go 14.8v like BFL states, you do not need a converter which drops to float or to 13.6v prematurely, you need a charger which can top charge the batteries in the time you have available to charge them. Run the MW at the same voltage was the PM to get the 75PM + 40 MW amps from 50%to 80%. Once the PM gives up, wait a bit longer and then crank the MW to the Max voltage and start watching that SG closely.
Quit worrying about how vigorously the batteries bubble and if they stink. If they are not getting super hot( 120F is the Mex imposed limit), keep going and quit *****footing them. You are trying to see how high you can get the SG, not determining how much bubbling makes you personally uncomfortable.
I personally believe taking a battery to 50%, then feeding it high current until 14.8v is reached, and held at 14.8v for several hours while current tapers, dictated by the battery itself at the voltage supplied, is like a slap across the face of a lazy misbehaving battery.
I slap my screwy 31 daily, and the Mofo seems to be a masochist. Every two weeks or so I slap it a little harder, and it behaves properly for a while there after. If I did not slap the SOB, It would have been recycled by now, instead of standing straight and giving its all night after night. Feeding it anything less than this, and it can't keep up.
I used to try and baby my previous pairs of 27's. They were like trust fund brats that could not work when asked, and just gave up prematurely. My single 31 is working hard, and being charged upto high voltages daily, and is acting like a proud self made man, as opposed to the simpering lazy trust fund brat.
Drain those GC's down, then spank them hard to ABSV with the PM and MW. Hold ABSV for an hour or two, then spank them again with 15.5x+ until SG no longer rises. Then you can watch your trimetric hold higher voltages during the same discharge cycles.
The 4 day slow discharge to 50% is going to be more detrimental than high recharge rates and high voltages. I'd be looking at ways to charge daily rather than the low and slow to 50% then wondering why they never seem to get near to 100%, even when the effort is applied.
