Bobbo is right - Pulling the fuse should remove the need for a charge line.
However, there are a few vehicles out there that when flat towed with ignition in ACC position, will draw down the battery no matter what you do. One way to make sure your vehicle is not one of them, is to do a test like I did: Put vehicle in neutral, ignition in ACC position, turn everthing off inside just like you would when you're hitching it up to tow. Then, attach a clip-on ammeter on one of the main wires going to the battery, watch it for a minute or two and see what kind of amp draw you get. If its less than 1 amp, you're probably good, and don't need a charge line. Any more than that, and you might should consider one.
I found with our Ford Fusion Hybrid, there was an average of a 3 amp draw ALL the time. Something about the electronics on Ford's hybrid vehicles, there are some electrical components that stay on when ignition is in ACC position, that draw down the battery. Only solution was a charge line, so thats what I did. 'Tis a very easy and inexpensive thing to add, though, if you do need it.
As to how to wire the charge line: Yes, tying into the 7 round plug on the MH will work, and is the easiest if you have such plug. I used 8 gauge wire IIRC, would not recommend going any smaller. Also, make CERTAIN that you install a fuse on that line next to where it connects to + terminal of your toad battery. I used a 15 amp fuse IIRC. You absolutely need to fuse that line, to protect it. A diode, like WyoTraveler mentioned, isn't a bad idea, either, although I have not done that.
There is also a few products you can buy that provide toad battery charging for you. I like
THIS one from RVibrake. A little bit more expensive than just simply running a line, but this unit would manage your battery's charging much better.
Will and Cheryl
2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook")
2018 Jeep Wrangler JK ("Wilbur")