[Al Skierkiewicz]

First things First...DO NOT SHORT THE BATTERY TERMINALS TOGETHER! Explosion may be the result.

Everything that has been described thus far leads me to want to look at the mechanical load. But let's clear up a few things first so you won't be mislead by the data. It is illegal (under First Rules) to connect two motors of any type to a single speed controller, but it won't produce the problem you have. Running any of the Chalupa motors, no load, will cause brown outs when switching between forward and reverse. Remember that a motor that is not moving is "stalled" and stall current is 134 amps for the small Chalupa and almost 100 for the large. Changing direction on any motor runs it through the zero speed condition of stall. If your battery leads are hot but the leads from the main breaker to the breaker panel are not, you likely have a loose connection on the battery, damaged contacts on the battery disconnect or a short to frame somewhere ahead of the panel. Since you are showing a breaker fault on the panel but nothing else seems to be a problem, discount the red light light as a bug in the panel. What that leaves is the probability that your two motors in the transmissions are running in opposite directions. You must check the direction by disconnecting one of the motors and giving the robot a forward command. Check the direction then disconnect the first motor and reconnect the second and check direction again. I believe the large and small Chalupa motors have a different direction when fed the same polarity voltage. The small Chalupa is counter clockwise. Since the current draw and battery voltage anomalies were nearly identical with the robot off the floor, everything is pointing back to the transmissions/motors. BTW, when the motors are fighting each other (going in opposite directions) there is a very distinctive and unnatural sound generated in transmissions.
Remember that the RC cutouts and goes to backup battery when the main battery falls below 8 volts which will occur during heavy loads. The main breaker, although rated for 120 amps, can withstand 600% overload for several seconds without trip. Similar trip characteristics on the 40 amp breakers but they get very hot after repeated tripping. A four motor drive with all motors running near stall will easily draw 400 amps.
Contact me via PM and I will give you my eamil address in case you have a problem on Thursday.