Well it takes 4 batteries total, the model # for the military batteries is 6TL so you can have interstate, napa, pep boys, auto zone, advance auto whoever cross reference them from thier catalog, and yes heed the above advice they are wired in series, positive terminal to negative. If you go to charge the batteries disconnect the cables from each separate one you are trying to charge. Just take a picture or make a mental note of the cable routing from battery to battery so you do not get confused.
Also I would not advise trying to jump start it either from a car or truck, either charge or change out the batteries. See the stick note under the duece forum for big time great hints on things to bring when picking up your truck..
By the way WELCOME!!!!! to SS and You have a great looking M817!! Congradulations!!