The alternator won't overcharge them. The voltage regulator switches the alternator on and off to maintain a "12v bus" voltage of between (usually) 13.5v (switch the alternator on) to 14.5v (switch the alternator off). On and off, on and off. The end result is a bus voltage that hovers around 13.8v - 14v.
In fact, don't count on the alternator setup to actually get those batteries up to a full charge. At the voltage your "12v bus" is being held at by the voltage regulator - it could take a couple of long days of driving to get a pair of T-105s totally topped off.
But yea, use the alternator by all means. If you're driving anyway, might as well dump something into those batteries. You can always fire up the gen and the good charger to get them topped up later, and it will take less gen run time if they're part way there already from driving.
Last edited by dwh; 07-03-2012 at 08:01 PM.
Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker