20A (it's not the truck, it's the Bargman cable)
Your limiting factor is the very thin "trailer battery charge" wire inside the Bargman Cable. If you bought the 60A version, you would overheat this wire and burn the insulation. The burn would possibly also extend to include others wires and the main sheathing as a whole, destroying the cable. That is probably also be true of the 40A version.
The Renogy supports multiple types of batteries (SLA, AGM, LiFePO4), and you can separate your "battery upgrade" project from your "charge from the TV" project. I did a home-built "charge from the TV" upgrade several years before I replaced my SLA batteries with LiFePO4, and I recommend that you do your Renogy project first. The Lithium Upgrade (new converter, bigger wires inside the Trailer, Coulomb-Counter Monitor, and maybe an inverter) is a bigger job.
My "home-built" is much more complicated, and roughly equivalent to the Renogy 40A. But it solves the Bargman cable issue by boosting TV Voltage (only on the the "Trailer Battery Charge" circuit), from ~13V to exactly 36V, and then uses an MPPT Solar Controller to drop the Voltage back down for batteries. It took about 3 hours of work in each vehicle (my 4Runner TV, and my Trailer.) You can look at my previous posts, if you're interested in duplicating that method. A Solar system with MPPT is pre-requisite.