Just a late update - I installed this Go Power converter:
https://www.amazon.com/gp/product/B007GN4T2E/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1
I left the WFCO in place (switched off the breaker for the converter). I put the Go Power in an extra battery box on top of the batteries on the tongue and routed the wires through the bottom of the box and the 120v cord up through the wiring opening on top. I’m looking for something a bit more water tight, but this is working great for now.
The new converter is working great - much better charging times for sure and gets full voltage on the Honda 1000. It doesn’t seem to hurt anything to have other low-powered items (120v) going as it’s charging (I’m using 2 extension cords - 1 to charge and 1 to run 120v to the trailer when needed. We just upgraded to a 2000 watt gen, so now I wish I had upped the amps, but this should be sufficient. I’m planning on getting some solar for summer camping anyways.