The truck is 12v. The two batteries are wired in a parallel circuit. This doubles the amperage but does not affect the voltage (12V). Yes, having a charger on one of the batteries will charge them both however, the best method would be to install the positive lead on one battery and the negative on the other.

I would certainly start by disconnecting any aftermarket accessories (your existing charger being one of them) and go from there. Finding a parasitic draw is relatively easy if you are familiar with the basic functions of a multimeter.

I started small with a few customers and it’s just grown by word of mouth. I run my shop part time after hours and on weekends, I do not have signs and do not advertise. If you’re good at what you do and do a little advertising you’ll be able to make a decent living after some time.