Voltage= Amperage x Resistance:
https://en.wikipedia.org/wiki/Ohm%27s_law
Batteries in parallel stay 12V but the amps available double(assuming both are same).
If one drops it gets refilled by the other.
Considering that most of the resistance is in the discharged battery and it has a much smaller mass and electrical potential than the two in the truck, any resistance in the cables/connection should be negligible.
The limitations are in how fast the dead battery will accept new electrons.
Resistance does play a larger role at higher voltages, but this is only 12V.