You can take power for the towed charge off the AUX wire, with a relay to the IGNITION wire, The AUX wire is not hot when you are starting the RV, it is when you are driving it. (This is the power to things like the radio).
The ignition is hot when the RV is running and starting..
Or take it off the battery with a relay to AUX for that matter.
Or use the Toad-Charge device, that is the best option.
Home is where I park it!