There is definitely something wrong on the truck side of your setup. Lots of folks have "charging while driving" issues and there are products out there to help, but they all still expect that you get 12V on the 12V pin. Sorry, this isn't a Heartland-forum issue. You either need to diagnose this yourself or get your vehicle to a mechanic or the dealer.
My totally uneducated, no-facts-included (working just with what you posted) guess is a short somewhere. Mice chewing through wiring - that sort of thing.
Before you go further, what exactly do you mean by 12V pin? There is a pin that's (normally) supposed to provide 12V for "charging" but it's usually called "AUX". I'm being hyper-specific because a very common mistake is to measure one of the lighting/brake pins instead. Vehicle manufacturers install these jacks all kinds of ways, including upside-down, and it's very common to have a lower voltage on a lighting or brake circuit. Your truck could even be mis-wired. If the voltage changes at all if someone steps on the brake pedal or turns on/off the lights (full ON/OFF vs AUTO) that's a clue.