Try it. If your going to toss them otherwise you have nothing to lose right.
Just don't blow them or yourself up.
90amp hour lead acid batteries should be ok with normal car battery charger.
If they are just normal sealed lead acid batteries they should be just like any other deepcycle lead acid battery basically.
Don't charge too fast, being sealed you can't put back in any water you boil out. I would not equalize them for same reason, although if your going to toss one that is not working well then might as well try it anyway before tossing.
At 90amps/20hrs =4.5amps for a C20 rate.
I would wire 2 for 180amps and charge at 10amps, or better 3 for 270amps and still charge at 10 or 12amps. Slower charging gives sulphates more time to disolve and helps alot sometimes, I would pulse charge them if you can to disolve sulphates also. Pulse charging is not the same as equalizing and should not boil out water.
Depending on the charge rates available to you, alot of chargers seem to vary, I would try for about 30hr charge period to bring them back up. 2amp is probably too slow, 45HRs per battery, 6amp is too fast at only 15hrs per battery.
Unfortunately today there is no such thing really as "a normal car battery charger" if you go to the store and buy a new one. There are the older kinds (if you can find them), Truely Smart Chargers, Brain dead chargers reffered to as Smart Chargers etc...
The old style, connect it, turn it on, it works. It tries to work even on a junk battery. No specail features normally, just straight amps like 2/6/12 or such.
"Brain dead" Smart Charger like the Vector 40 amp I had, good for alot of things, but if a battery was good but DEAD it would give an error and not turn on untill I charged the battery some other way a bit first. Then it would work. It had a pulse feature to desulphate.
This type probably will not work for you since it probably would not turn on for your dead batteries. Even if the batteries are still good the charger may show they are junk just because they are dead!
Black & Decker "truely" smart charger (made by vector) will turn on even on the worst batteries. Will flash an error message but tries to charge even the deadest of dead batteries. I used this on several cars that sat many months-year and the battery had NOTHING and this charger started at like .2amps and slowly worked it's way up as the battery would take more charge till it got to the 10amps I had it set for. It was flashing an F02 or such error at times on the charging amps display, but it worked!
Has modes for Normal Lead acid, Gells, and AGM types. Has desulphate mode, equalize mode, and other nice features.
If you need to buy a charger I would recomemend this type. I paid $45US for my 2/10/15amp charger. The only smart charger I have had that will at least TRY to work on ANY battery so far.
If you try a "Smart" battery charger and it will not try to charge your batteries or it shows an error code saying the battery is junk, try a different charger!
Also sometimes you can "trick" the braindead stuff to work by connecting a charged battery for a few minutes to the dead one then take the charged one back off once the charger is working a few minutes and then it will stay on.
If you have a bunch of those batteries you should be able to save at least some of them I would think, maybe most of them. It may take a bit of time and playing around, but if they are not physically damaged like shorted you may be ok.