Sounds like you know for sure the PD 55 amp is what you want. It would (somewhat) be a better setup with the larger wire running behind the couch. The reason I only say "somewhat" is that you'll be reducing voltage drop, but the voltage on the PD charger isn't adjustable (AFAIK), nor temperature compensated. So, while you will be getting closer to matching the charger's output voltage with what the batteries see (i.e. less voltage drop), the voltage output of the charger isn't necessarily going to be what your batteries want. That depends on which batteries you have (their charging specs) and how close you keep the batteries to 77ºF. 77ºF is what chargers "assume" your batteries are at all the time if they don't have a temperature probe.
So what I'm getting at is that yes, larger wire and a shorter run are improvements. But like an anchoring setup on a boat, it's not just the anchor, but also the line, chain, shackles, attachment to the boat, etc. Similarly a charging system isn't just the charger/wire. But anyway.
The charging "loop" (which is also the load loop on the Chinook as they made it do double duty) consists of the following:
1) The positive wire from the charger to the LVD.
2) The positive wire from the LVD to the house batteries (goes out under base of pillar behind LVD).
3) The positive jumper wire between the two house batteries.
4) The negative jumper wire between the two batteries
5) The "chassis ground" wire from the battery negative post to the frame.
6) The "chassis ground" wire from the frame to the charger (this goes from under the shower-ish to the brown box).
To completely upgrade the wire to 6AWG, each of those legs would want to be at least 6 AWG. Then you can up the fusing to 60 amps (presuming all components of that circuit are up to that ampacity). Fusing coming off the battery should have an AIC (ampere interrupt) rating sufficient for the short circuit rating of the battery bank (I don't know what you have for batteries so can't advise). If the AIC rating is too low the fuse body can just self destruct regardless of the fuse size. Today's larger banks can really pack a wallop. For example two Group 27 Lifeline AGMs have a potential short circuit current of 6,748 amps
A lot of typical fuse types are only rated to 5,000 amps.
The original wire that did this function (carry charging current to the batteries, plus carry battery power to the loads) was 8AWG, and ran from the "brown box" over to the corner by the shower, up to the area above the long window, OVER the long window to the overcab, then down the pillar to the LVD. Geez, could they have made it any longer?! 6AWG running along the couch wall will be heavier and shorter; but just do all the legs and not only the one - it's not great practice to have varying gauges of wire in one wire run. Also of course protect and secure the wire under the couch.
I don't know much about the rest of your system or use case based on this thread, so it's possible some of this will not apply to you.