Ben,
My trailer came with a WFCO converter as well and I could never get it to properly charge either. Wire size was part of the problem, but not the whole story. As others have suggested, you may have better success by putting in a different charger.
I decided to abandon the charger portion of the WFCO converter and just use it as a power distribution (AC and DC) panel. To do this, I removed the AC input to the charger portion of the converter and the DC output fuse (prior to the smaller distribution fuses). I then put a new charger, solar charger, and inverter under the bed in the front of my trailer with parallel 1/0 cables (~4/0 equivalent) to my batteries, and ran two 10 AWG (I have a 30 Amp system) cables up to the new charger under the bed with a Levitron 1288 DPDT (center off) 30 amp switch to serve as a manual transfer switch (so I can power the whole trailer with either shore power or the inverter).
Unfortunately, it's hard to do all this in small steps. Assuming you have the space close to your batteries like I do, you could just add a new converter with a heavy cable to the battery, and a transfer switch with the two AC lines back to the old converter and save the rest for later. If you never plan to have a whole house inverter, you could skip the transfer switch and just run one fused 120VAC line to a outlet at this location that the new charger plugs into.
Hope this helps,
Steve
