Trailer battery connection

I have a trailer with an internal battery to run lights, a winch, the trailer emergency brake, that sort of thing. My previous tow vehicle had a charging lead so that the trailer battery was charged by the vehicle while it was being towed. The current tow vehicle doesn't have this lead connected, so I charge the battery while stopped with a normal battery charger.

I would like to connect up a charging lead, but I wonder if it should be connected directly to the vehicle battery, or to a switched source? My last truck seemed to have it direct, since I could run the trailer lights by plugging into the truck, which would be handy.

Any ideas on how this is normally wired would be appreciated.

I have a similar setup on my trailer.
I use a solar batter charger permanently mounted on the top, feeding the batter.
Free energy, and the battery is always hot. If I run it down a lot using the winch, I'll use a regular charger to restore the charge. Normal usage the solar panel keeps up with.
Rex in Fort Worth