Kaz,
You keep referring to batteries as plural. If you have more than one and they are strapped in parallel, that is part of your problem. Lead/Acid batteries do not like to play nice in parallel. If you are going replace the bank, get a pair of golf cart batteries. These are designed to be abused by people that are more interested in playing golf than taking care of the batteries. Two in series makes a nominal 12V and they compatible with everything you have and even if you drain them real dead, they will stay and play at least a while.
If you do drain them dead, start the recharge out slowly and when they get their breath back then you can charge them faster.
Matt