We never leave our tow vehicle connected to the trailer when parked, even if we're not physically disconnecting from the ball. We do not want to charge or discharge the tow vehicle's battery
from the trailer unintentionally.
We do have a dash board switch in the tow vehice that allows us to disconnect the tow vehicle's battery
from the trailer. For us connecting the tow vehicle's battery
to the trailer's battery is an intentional act.
The wires from the Converter to the battery are pretty substantial because they have to handle high currents, really much higher currents that the panels produce..
In retrospect it would have been better to wire the solar panel
to the input of our Inverter. The wiring from the battery to our 1200 watt Inverter has to be capable of 100 amps.
In general the voltage drop caused by solar panel
current is relatively small because the currents are relatively low. As well the distance from the solar
controller to the Convertor are short. The distance from the Converter and Inverter to the battery even shorter.