Use the same battery to OEM fuse panel (which the OEM converter is also connected to) wiring, with your new battery bank as is there now with the old battery. The wires have enough ampacity to run the 12v rig systems so no reason to change that.
Use new fat wires with your new converter up front to the big battery bank to take the high amps of the new converter. Say three feet each #4, but IMO use #1 AWG to get the most out of the deal. Don't need fatter.
You don't need to rig a 120v receptacle up front to run the new converter since it will only ever be needed when off grid and then you run it by plugging it into your gen (via extension cord as required)
Put your solar controller up by the new battery bank and you can share the fat wires from converter to battery with the controller's output to battery run. Just go from controller output to new converter (when not a portable!) and then share from converter-battery.
If you have an inverter, now you put that up there too and the fat inverter-battery wires can also be the converter and controller wires to the battery by using a positive buss to collect all the pos wires and then a single fat wire to the batts. The negs all go to the Trimetric shunt or maybe a neg buss first if too many lugs to stack, then single fat wire from buss to shunt and from shunt to batt.
Don't forget, if you have a shunt, to move the existing battery to frame neg "ground" wire from the frame to the outer end of the shunt and then just have the inner end of the shunt to neg batt post.
BTW, your gen will run this PF corrected 100 amper which can be set to 14.8v (the usual desired absorption voltage for doing a 50-90) by using the pot in the little hole in the side as shown. (or any other desired voltage that meets your battery specs) So it works like the one in my photo above. Check the price compared with that PD 80 amper or whatever you are looking at now. ( However, as said, whatever floats your boat)
https://www.amazon.com/Powermax-PM3-100LK-Power-Supply-Light/dp/B01MS4F5QT/ref=sr_1_13?ie=UTF8&qid=1521245823&sr=8-13&keywords=PowerMax+converter
