If you are going to boost the existing charge line.... start with checking the fuse rating. I would go with output at about half the fuse rating.
I feel that to be excellent advice from time2roll.
Mike - if you go "too big", your most likely disaster will be melting insulation within the 7-wire trailer cable into the Tow Vehicle "Bargman" connector. Your second most likely disaster will be an overheated "Trailer Battery Charge" wire (TBC) within the tow vehicle itself, between the fuse block (under the hood) and the Bargman connector in the rear.
Even though the TBC circuit is usually fused at either 30A or 50A, automakers tend to put very small wires in their vehicles. In my older Toyota 4Runner, for example, it was fused for 30A - but it appeared to be #14 wire at best. It might have been even smaller than that. I rewired it to 10-AWG (as part of a complicated "Charge from the 4Runner scheme" involving TBC Voltage Boost (to 36V), the Trailer auto-detecting high voltage, and then switching the TBC from the "12v" system bus to become input to an MPPT Solar controller).
The 30A fuse would have implied 10-AWG, but Toyota used (IMO) unjustified "chassis wiring" rules in declaring the small wire to be "adequate". In my opinion, the long wire (perhaps 10-11 feet long) is not chassis wiring, able to dissipate heat at its connection ends -- it's too long for that, and generated heat (from high current levels) MUST escape through the insulation jacket. At 14 AWG with 90c insulation, and 90c, I would have rated it no better than 25A -- but again, it appeared to be even smaller than that. And the Trailer cord is much worse:. The TBC wire within your trailer cord is probably even smaller, and the cord assembly wraps all the wires in a heavy rubber cable - providing too much thermal insulation, and keeping the heat inside to start melting things.
Even without risking fires or melted and broken wires and cord assemblies, you would waste a ton of power by trying to push 30A through wires that small. So - exactly as time2roll says, I would not purchase any in-trailer DC->DC battery charger at a size which would pull more than HALF of the TV's fuse rating for the TBC wire.
That's the low voltage side, typically starting at roughly 13.6V in the engine compartment and suffering "Voltage drop" along the TBC wire and ground return from the Trailer.
Example: In order to charge an LFP battery at 20A and 14.2 Volts (284 watts), an 85% efficient DC->DC charger would need to pull 334 watts from it's input side. Since the total length of TBC (one way) is 20 feet, and the average size of TBC and Ground wire is probably not better than 14 AWG (the grounding wire is bigger, the TBC is smaller), voltage drop is almost 21%, yielding a voltage of only about 10.6V at the DC->DC charger. You simply can't do it - the 334 watts would need more than 30A at this low voltage, and it would blow a 30A fuse.