Moving Lithium batteries to under bench seat

+1, you couldnt depend on a wiring diagram even if there was one. Use a multimeter and check each of the wires with the continuity setting to be sure you know what is running where.

BTRice: how did this turn out?  I'm in the process of ordering a LiFePO4 battery for my 2021 193 that I'm planning to (try to) put under the front bench.  Based on my experience with my rpod build quality, I'm NOT looking forward to the rat's nest of wiring I expect to find.

You do want to install a converter configured for Li battery charging. There is too much risk from overcharging Li batteries. Placing them in a temperature regulated space is a good idea, Li batteries don't like to be charged at low (below freezing) temps.There are now self heating Li batteries available now but why waste energy?

The issue regarding charge profile for Li batteries applies to all your charge sources, including solar and tow vehicle charging if you plan to use those. Most solar charge controllers these days have a Li setting, but proper charging from an alternator/TV battery requires installation of a DC DC converter.

That should connect directly to the tow vehicle 12V charge conductor as exits the 7 way connector, and no trailer circuits should be connected directly to the TV 12V supply before the DC DC converter, with the exception of the turn signals, marker, tail, and brake lights, which are never connected to the trailer battery.

The interesting execption to this is the electric trailer brakes, which are normally supplied from the TV battery via the brake controller, unless the brakeaway pin is pulled in which case they switch over to the trailer battery. This circuit shouldn't need any modification when converting to Li, just thought I'd mention it because of it's unique status.

Generally speaking, Li batteries don't need a multistage charging profile, unlike lead acid batteries. PbA batts do because their internal resistance increases dramatically as they near full charge, so you have to taper the current to get them there. They also need a small continuous (or at least frequent) float charge current or they will self discharge and the individual battery cells will become imbalanced. The continuous float charge corrects this by slightly overcharging some cells, allowing the others to get to full charge. Finally flooded PbA's need an occasional equalization charge at a highrr voltage to mix the electrolyte, which tends to stratify (sulfuric acid is heavier than water) otherwise. Simply put PbAs both tolerate and need some overcharging to perform properly and last.

OTOH, Li batteries can simply be charged up to a set voltage and stopped unless you are trying to recharge them at a very high rate (like in an EV fast charge station). It is harmful/dangerous to charge Li batts higher than that. That means a different approach has to be taken to assure cell to cell balance another approach is needed. This is an electronic cell balancer which should come integrated into any Li battery you buy. These usually (but not always) also come with a high voltage cutoff to prevent overcharging in the event your charger malfunctions. Some also monitor battery temp and limit charging when the battery is too cold. So simply put Li batteries don't tolerate or need overcharging.

I'd suggest that you let the charging revommendations spec'd by your battery supplier set the direction you take for the charger(s) you get.

Re WFCO, I am one who has had my lead acid batteries overcharged by a failed charger. Mine went to around 16-16.5V and stayed there. Luckily I had flooded golf cart batteries, happened to hear and smell my batteries overcharging, found my battery plates to be nearly exposed,   and added water in time so no harm done, but it couldn't have been more than a week since I'd last looked.

If those had been Li batteries without internal over voltage protection they would almost certainly have been destroyed or caught fire.

So I don't trust WFCO chargers personally, and I also wouldn't install Li batteries inside the trailer unless they were internally protected. Too much fire risk for me.

I think a 20A DC/DC is fine. Note though that it will continue to try to charge your trailer batteries even after you shut off the TV, unless you disconnect the 7 way or add a relay energized by the TV ignition circuit to automatically disconnect for you. Stopping for a few minutes to fuel up will be fine but if you stop for an hour or two or longer on the road I'd be sure to disconnect.

Disconnect or cut the +12V conductor on the trailer side of the 7 way before it connects to anything else and run that to your positive DC/DC input. Then run the dcdc + output to the trailer side of the line you disconnected and you should be fine.