At this point it is customary :) to ask where you are taking the voltages and with what. You need the voltage meter to be at the output of the converter, not at the battery. This is because there might be an "open" between the converter and battery which is not the converter's fault.
Check the reverse polarity fuses on the converter or its DC fuse panel wherever they are located in your rig. with a new battery, there could have been an "oops" at first in installing it. Lights etc will still work but fans will twirl the wrong way around.
Then there are the usual battery disconnect switches and battery fuses or circuit breakers to check if there is 13.6 at the converter but only 12.x at the battery.
Whether to change out the WFCO depends on your camping scenario. If it is working and all you do is use shore power then there is no reason to change it. If you do a lot of off-grid camping, then there are several options for what to install or use instead--including just leave the WFCO in place and use a different charger just for that fast charging 50-90 job.