standard charger for a stock 20m is 12v 25amp incredibly expensive
measure the voltage at the point the charger connects to
sounds like yours is not stock
do you have 5 contactors along top of "door"
slow drive is very common
open "door" jack the back of truck until drive wheel is clear of ground, bridge out door switch
drive truck at max speed and confirm if all 3 left side contactors close plus either forward or reverse