my question is a follow up to the Are you sure they are 12V?
Are you sure they are the chassis battery. Most of the diesel engines I have seen whether they are diesel pickup, Rv, Semi Truck they have at least two 12 volt batteries in parralel for the starting batteries.
A lot of Rv's use 6 volt batteries in series for the coach batteries to run the 12 volt items in the motorhome itself ( for example interior lighting)
If you replace them make sure to hook them up the same. 12 volt starting batteries are hooked in parralel ( plus to plus , negative to negative) hooking them up in series ( chassis ground to neg on battery 1, positive terminal of battery 1 to negative terminal of battery 2, Positive terminal of battery 2 to plus wire going to engine) if you hook the 12 volt batteries in series bad things happen because it now becomes 24 volts and you start frying engine computers, dash computers, gauges, sensors etc.
6 volt batteries hooked up for the coach are in series to get 12volts. Basically the rule to follow all 12 volt batteries are hooked up in parallel if you are hooking up 6v batteries there will be two batteries hooked in series no matter how many batteries you have. if you have more than two 6 volt batteries they will be hooked as two in series and those two will be hooked in parallel with 2 other 6 volt batteries. so 4 6 volt batteries will be hooked up as battery 1 and battery 2 will be in series, battery 3 and battery 4 will be in series. there will be cables hooking battery 1 for example in parallel with battery 3. If you did not understand what I just wrote have someone hook them up for you.