If you have 1 sb (PLASTIC) connector on 1 charger that has a bent tip or cracked housing, that poor connection on the charger side will cause the connector on the battery side to melt the part of the connector that holds spring pressure (opposite the spring), this in turn will melt the one that connects on the truck, then a new battery in the truck will melt the same section. Take a good look straight down into the connector side and make sure that every SB connector in the place has a good (flat) piece pushing on the tip and it holds good spring pressure. you say they are good, but ANY crack in the housing makes it no longer good, even if it may appear OK all the rest of the way.
You might want to assign one charger to only a couple of batteries for a while to see if this is not a "growing" problem.
You may also want to (should) insure the chargers connectors do not fall on the floor when disconnected, as they can get run over, and look OK externally but have cracked and weakened the part that holds pressure on the spring and contact.
Almost all industrial batteries over a few weeks old will have SOME continuity from the posts to the case. Heck, your body has SOME (that is what a lie detector reads, although they like to refer to it as "galvanic skin response"). You would have to keep the battery top -very- clean and -very- dry not to have SOME. Different controller manufacturers have different allowable maximums, GE controllers say the maximum is 100K ohm, and any less resistance should be dealt with before any sort of troubleshooting should be attempted.
If you ever read the battery manufacturer's instructions that come with a new battery, you may notice they expect a daily charge cycle*, fill with water ONLY -after- the charge, wash off the top and frame with clean (and properly disposed of) water and allowed to air dry. The manufacturer of your car also says (in the owners manual) to check the motor oil every time you add fuel, and I don't know too many folks that do either one. but a good wash and air dry every so often will sure help.
*= a proper charge cycle being 8 hours charge, 8 hours cooling down and 6 to 8 hours operation (we know you don't really drive the lift all 8 hours in an 8 hour work day) with one of every 5 of those cycles to have a 15 or 16 hour charge often referred to as a weekend or equalize charge.