[DaveNP]

Start by saying I'm not a caravaner (so shoot me if you want I just browse here occasionally to broaden my mind), however I do drive an articulated truck with a big trailer.
Trucks do use a 24v system so dragging up elementary electrical formulae from my school days, twice the voltage means half the current which would be 1/4 of the power lost due to resistance, but with a 40ft long trailer we still get lots of power to the rear lights, I'd suspect that losses due to length of wire is minimal unless the builders have really cheaped out and used very thin wires. What I find is more often a problem with an artic is the quality of the connection of the plug/socket on the front of the trailer, often one can observe that with the taillights and fog lights on adding the hazard flashers will cause the other lights to pulsate due to the earth return not being able to carry all that current, usually on the truck that is down to damage and wear in the plug/socket, on a caravan I would also consider the possibility of corrosion in the plug/socket reducing the efficiency of the connection.

Batteries on truck trailers are not so common but with more and more electical gizmos on the trailer they're not so rare either. The charging methods are perhaps a little behind caravans given that it's not so established. Early trailer batteries were often only charged when the side lights were turned on, in a crude sense it ensured the truck was up and running before putting extra load on the electrical systems but it did lead to one company I worked for having to make a ruling that sidelights were to be kept on at all times as over the summer the batteries for the security systems would go flat leaving the trailer 'insecure'. My present job involves some double decker trailers with a moving deck which is done by an electrical hydraulic system, these are generally powered from mains electric at the hub but use their onboard batteries at the outbase depots, usually there's no problem but if we need to we can take power directly from the tractor unit but that uses what we call an Anderson Lead which has wires like jumper cables and a chunky plug with large electrical contacts.
As others have suggested the 80% charge figure may be more related to the way a battery charges combined with the level of sophistication of the systems in the car and caravan to manage when the battery is actually being charged. Bear in mind that on a car we don't tend to do a deep discharge of the battery very often, usually just enough to start the car, whereas with a caravan one would be discharging the battery all evening to have light, watch telly etc and then need to put back all that charge while driving.