you believe this?

i have made various posts here about my truck and trying to fix an electrical problem with it. i have made some headway, but the thing i found last night is just amazing. i have a digital cap in the bed of the truck that has a voltage readout. with the truck running, i get a readout of about 10.3 volts (should be over 13). i accidently sat on the tailgate while i was working on it last night..and the volts jumped to 13.5 (where it should be) and stayed there! stood up and the volts dropped back down. why would sitting on the tailgate make the voltage go up? think maybe a wire is getting pinched somewhere? i looked all over..cant find one, but may have to dig deeper.

also, another question. i have 2  batteries in my system. should the ground for the second battery directly attach to the first? or just each battery has a separate ground to the truck? or should the grounds connect to each battery AND each has a ground to the body? couldnt find a post that really said

thanks

run a nice fat wire from your bed, to your frame. , ,and batt to frame.     Whats happening is that  the connections between your bed and frame  are probably rusted, not allowing the power to pass properly.  The main batt is grounded to the main body so everything in the main body is probably un affected, , but the bed is separated from the main body.

Know what im saying?