WFCO--World Friendship Co?--first time I actually knew what the WFCO stands for. In general, most everyone who has mentioned having one of these with problems has been told to get a Progressive Dynamics (PDI) Intelli-Power converter. Just seems to get a regular recommendation on the internet forums.
However, your current converter should be enough if working correctly, and the battery(s) good, to take care of charging.
As mentioned above, maybe you are using most of the converter output in the trailer instead of going to the battery. Maybe you just aren't leaving it on long enough. Maybe your battery is not in good shape. Lots of things to check/find out.
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died