[bobofthenorth]

If I hook up a battery charger to the one battery under the hood, will it charge the other battery under the floor on the frame rail?

Yes but ..........

The remote battery will see the voltage at the end of its long cable so the connections on either end of that cable are doubly important. If one or both of those cable ends is compromised by corrosion the remote battery will charge more slowly and won't deliver as much fire to help get the van started. And don't neglect the grounds. The little tiny electrons need to leave the battery AND return to the battery. Anything slowing them down will affect performance of the whole system.

 

[bobofthenorth]

I've seen that "balanced" charge diagram many times and its nonsense every time. It assumes an inadequate parallel cable. If that is in fact the case then the cable is the problem, not the charging lead arrangement. Use big cables. Keep the connections clean. If you don't believe me, measure the resistance between the positive clamp on Battery A and the positive clamp on Battery B (or C or D or whatever). If you get a measurable resistance then you don't have large enough cable or you've done a bad job putting the ends on the cable you do have. Note: don't measure from the battery post - measure cable clamp to cable clamp. If you've got resistance between post and clamp you just need to clean it up.

Here's a tip that actually works. You can locate high resistance connections using an infrared thermometer. Put a heavy draw on the system - start it a couple of times with the lights on. Then immediately shoot the temperatures of every connection. The hotter ones have higher resistance. In a perfect world they'll all be cold at the same temperature.