You can use the (+) 12 v pin of the seven pin hook up as a charge wire from the RV to the towed car's battery. Just install a couple of auto reset circuit breakers, (20 amp) one in the RV to protect the wiring from the battery to the plug, the other in the towed car between the plug and the battery. When both batteries are charged up, very little current will flow from one battery to the other. When starting the RV, the starting current might be a heavy drain, but the auto circuit breakers would trip if excessive and reset when the RV engine starts and starts charging. I've done it in my towed car and it's never been an issue. I don't see the need to buy a Towed charger, the wire I described does the same thing.
Are you sure the dash lights won't go out after a minute or two, like the interior lights? Even if they stay on, the charge wire should provide enough power for the lights and the brake system.
If you've got multiplex wiring, a light bar is a simple solution. You might put the bar in the back window if possible, that way no scratching or chafing of paint by wires or magnetic light mounts.