You have changed batteries no change, you have bridged switch out no change, you will have to get your voltmeter out and check where you are loosing the voltage, you may have a bad connection somewhere check voltage across battery, check condition of battery leads for corrosion and oxidated terminals, check voltage at line contractor input side and output side, check for terminals being secure, change line contractor if suspect. Check voltage at fuses ensure there secure and finally check connections at power unit and voltage.