Hey, Way2Roll,
I have always had a charge line wired in with our towed vehicles, although our current one (Jeep Wrangler) doesn't use it or need it.
With a 4 flat connector, you are right, there isn't a wire to use for charging battery. You will need to switch to a 6 round setup, as the 6 wire setup does have an extra wire you can use for the charge line.
You don't have to spend tons of $$ and install a battery charge kit or anything (I guess you could if you want, but I didn't). I just ran a charge wire from the Motorhome to the towed vehicle's positive terminal of the battery, I believe it was 14 or 16 gauge. Just make sure you install an inline fuse or circuit breaker on that wire as close to the battery as possible!
My previous towed vehicle (2014 Ford Taurus) would have a dead battery after just a few hours of towing. Charge line solved that problem completely. Same thing with the hybrid Ford Fusion we had before that.
PM me if you need more specific info on how to do this. :)