Just wondering if instead of a DC-DC converter, could you not just fit a higher output alternator?
Lithium batteries, due to their very low internal resistance, will take all of the charge they can get. If they are low on charge, this means very high amp draws, which will damage wiring unless the wiring is upsized enough to handle it (we are getting into welding cable sized wiring) AND a large enough alternator is provided.
Some will argue that the charge wire in the tow vehicles wiring harness acts as a resistor, due to its relatively small size, and that no DC-DC charger is needed, but you are then getting less than ideal voltage to the trailer, and at the risk of the wiring harness in your tow vehicle being damaged.
The other issue is that new vehicles use computer controlled alternators. No more regulators on them, just the computer. It raises voltage after a start to re-top the battery, and then drops the voltage to about 13.0 to 13.2 volts, which is ideal for maintaining a lead acid battery and for operating the equipment in a car or truck. However Lithium needs 14 volts or better to get fully charged. Many truck computers will sense the connection to the trailer and not drop the voltage down, but maintain a higher voltage for charging the battery on the trailer (which may only be a small sealed lead acid break away battery on a flat bed trailer or the like). Fact is, tow vehicle trailer charge lines are not designed to charge massive RV battery banks, but rather to keep a break away battery topped off.
The battery to battery charger does two things, one, it limits the charge amperage, which in turn limits the draw on the tow vehicle system to something it can handle. Second, it is a buck/boost transformer, and can either lower or raise the output voltage as necessary. Most DC-DC chargers have a way of setting them for either lead acid or lithium so they know what output voltage to generate. Thus, if charging Lithium, it will put out a high enough voltage (14+) to actually top off the lithium to 100% or nearly so if charged for long enough, and if it is charging lead acid it charges at a lower voltage (mid to low 13's) so as to not cook the battery, but again, get it fully charged in a reasonable time.
The other nice thing about DC-DC chargers is that they almost all have a solar input and a MPPT controller, so, unless you are running a huge solar panel array, that can serve as the solar input also without the need for another box.
Note that if the DC-DC charger is say, 20 amp, that is its OUTPUT max. It will draw quite a bit higher on the input side to be able to generate the power and voltage for the output.
Charles