There are several factors in achieving a high charge rate:
-- Voltage: The textbook numbers for a 12v battery are 12.7v at full charge and about 12v when flat. Modern open lead acid batteries (e.g., Trojan) usually spec a bulk and acceptance charge voltage of around 15v at 70F. Modern AGM batteries (e.g., Lifeline) want about 14.4v at 70F. Both want more at lower temperatures. Don't look at the absolute voltage difference, but rather the percentage that these voltages are higher than 12v. You charge voltage is determined by your alternator/regulator set up.
-- Current: Lifeline AGM batteries spec a charge amperage of up to 5C. Some others actually spec no limit. Open lead acid batteries typically want a lower charge rate. The current available is determined by the size of your alternator and the capacity of your cabling to deliver it to the battery.
-- Time: The greater the voltage difference between the battery voltage and the charge source, the greater the amperage flow. As you might expect, as the battery voltage rises (i.e., as it is charged), this difference drops and the charge rate drops. Between this and surface charges, a full charge takes much longer than you expect, one reason why a solar kit or shore power is often needed to complete the charge. (Lifeline, for example, specs two additional hours of acceptance charge beyond "full" charge. That is, two more hours of high voltage/low amperage charge to get you past a surface charge.
-- If you connect your camper battery(s) to your starter battery(s) with a long, small cable and lossy quick disconnects, then your alternator will quickly recharge your starter battery(s) and effectively ignore your camper battery9s). If, on the other hand, you use an adequately sized cable, the camper battery(s) will quickly draw down the voltage of your starter battery(s) and your alternator will respond by producing current, up to the limit of its design, temperature, etc.
In any case, for most campers we are talking about twenty odd feet of wire running the length of your truck and passing through multiple bulkheads with a battery at each end; a potential arc welder under your seat. Fuse that cable at BOTH ends, as close to each battery as possible.
