Well, according to wire tables a #8 wire will carry 45 Amps, a #6 60 amps in free air. Of course thats at 110 AC. For 12 volts DC... who knows but I think it's safe to assume those tables are about right.
The negative side doesn't have to go all the way back to the battery. The negative side of the battery is connected to the chassis. You could just run the negative down to the chassis from the converter and connect it there and let the chassis carry the negative current load. The positve lead has to go back to + on the battery but instead of running all that wire can you relocate the converter closer to the battery or to the + fuse panel instead? No matter where the converter sits you can save wire on the - side by using the chassis. Remember too, that if that's the way you wire it you will also have to upsize the negative from the battery to chassis also.
I think another 8 is sufficient. 2 #8 = 90 amps capacity. If you have to run further then about 20 feet, upsize to a #6 to make up for wire resistance.
If you buy another 12 volt battery you will interconnect the positive leads of each battery to each other. The negative of the new battery to the chassis, or interconnect each battery negative, or both the negative to the chassis and the other battery. When using wires to interconnect batteries or to chassis remember to always use a large enough wire to carry the max load of all 12 volt stuff on at once. I like larger battery interconect wires. If you're going to pull as much as 60 amps through your rig because you now have a 60 amp converter, I would run at least a # 4 (#2 would be better) or larger wire for all your battery interconnections and grounds. In my rig they are all 4/0 but that's just me. I like not worrying about my battery cables ever getting hot. Any loose battery connections anywhere will cause these #s to be all wrong and wiring will overheat. A loose connection is like adding 500 feet of wire in the circuit. Check and recheck all wire connections, in your fuse panel, the batteries, in the 110 side, all negative and grounds, EVERYWHERE. A trailer is not like a house that just sits there. A trailer rattles and shakes over every mile and they do loosen connections.
Good luck and keep us posted. You will like the PD-9260. New technology that's far nicer to your batteries, very quiet, and much more efficient. Little chance of ever overheating and you can buy a pendant that will tell you what mode the converter is in or manually cycle them at your preference for another 10$ Cool!
-Paul R. Haller-