This
link will help you understand your systems better. The amount of charging you get from your tow vehicle (if it is set up to charge in the first place) is minimal at best due to the long wire run and small wire. Unless you are driving across Texas, you won't get much from charge from it (assuming it is set up and working) anyway. It is more of a battery maintainer. Start your trip with a fully charger and equalized battery for best results.
The guys on here are really helpful. Some just have different styles. No offense is meant.