The summary for you is: The 5er to cannot pull enough amps from the Truck to hurt the alternator. With no DC->DC charger, the maximum current will be severely limited by "Voltage Drop" within the connecting 7-pin cable (probably not even reaching 10 amps). With a DC->DC charger, more current would be allowed, but the DC->DC charger itself limits the amount of load (typically to 20, 40, or 50 Amps). Your truck might notice the addition of a new 50 amp load. Adding a DC->DC charger would increase the load on your alternator, rather than reduce it.
Very detailed answer follows:
The 7-pin (at the back of the truck) is supplied at roughly 13.3v - 13.5v while driving the truck (there are just a few seconds of higher voltage following engine start, to recharge the truck's own battery).
When the 5er is being towed, that same nominal voltage is present at the 7-way Bargman connector. Bur resistance, along the very thin "Trailer Battery Charge wire" within the Trailer's Bargman connector cable, will increase rapidly when high current (more power) begins to be pulled through the cable. With no DC->DC charger , this is seen as as "Voltage Drop" at the 5er battery pack. A Voltage which started at 13.5V will try to charge your battery pack, but only with a little bit of current. When the battery pack begins to pull current (and power) through the bargman cable, the charge current will basically pull that Voltage down to a significantly lower level (perhaps 13.0 - 13.2 volts, it depends on the amount of current), and the battery pack will be accepting much less input power at that relatively low charging voltage.
In short: Due to high resistance along the path to and through the Bargman cable, very little current will actually be flowing. Your alternator will barely notice, it's a smaller load than turning on the headlights.
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A DC->DC charger creates a higher load at the Alternator (in watts), by pulling more current through the Bargman path. The Voltage drop (power loss in the bargman cable) becomes larger. But the DC->DC charger boosts that lower ending voltage (for example, 12.2 volts, with 1.3 Volts consumed by "Voltage Drop") up to a higher voltage which the batteries can actually use for charging (13.1 to 13.6 volts).
The DC->DC chargers are willing to charge at up to about 13.6 volts (some are programmable), but they reach their output current limits (typically 20A or 40A) before the charging voltage into an "12V" LFP battery pack becomes that high. In most units, the "maximum current" value corresponds to the lower Voltage input side (from the truck). When pulling current of 20A through the Bargman cable's "Trailer Battery Charge" wire (and also "returning" the same current back to the truck, on the slightly larger Bargman cable "grounding" wire), the approximately 13.5 Volts available at the truck experiences Voltage Drop (due to resistive power losses with in the long wires). With a good and not very long Bargman cable, the end Voltage (at the DC->DC charger terminals) might be around 12.2 Volts. It varies a lot from one Trailer to Another, but remains constant for one Trailer-to-TV connection pulling all this current. In this example, the power input to the DC->DC converter would be 12.2 Volts times 20 Amps = 244 watts.
The DC->DC charger's voltage conversion process is not 100% efficient. If we assume 85% efficiency in the conversion, we have up to 207 watts available for battery charging. This required 20 Amps from the truck's alternator and battery - a trivial load, in comparison to the capability of your truck's alternator. A 40A or 50A DC->DC charger can do a better job of charging the 5er batteries, but costs more alternator power than a 20A version (and also risks burning out a wire within the bargman cable). Running without a DC->DC charger will consume far few Amps (from the Alternator) than installing any DC->DC charger device.
