[Tannaton]

Whether or not the leisure battery gets charged to more than 80% I think depends on the sophistication of the split-charge system in the car.

One primitive systems which were just a basic relay contact, its possible with the electrical loads in the car that if the nominal battery voltage was lower on the car battery (i.e. it was older or of a different type) then the caravan battery may not reach full charge. I don't think this is anything to do with the resistance in the wiring to the caravan as when nearly fully charged a healthy battery should be drawing less than 1 amp.

On more modern systems however with diode isolation and voltage sensing relays this is no longer an issue.

I personally cannot see any benefit from installing a wheel driven charging system on the caravan, it would be expensive and complex. There is no "live" axle to drive the alternator, so you would need to re-engineer the suspension. You would then need some form of "step up" gearing as alternators start to work in the range above 3,000 rpm. I doubt caravan wheels get much above 300 rpm (if you look on your car engine you will see that the crankshaft pulley is much larger than the alternator pulley).

Why go to all that trouble when it is being towed by a mini power station?

For those using or contemplating solar - the panels are getting better and cheaper BUT you are much better off spending a little less on the panels and getting a proper MPPT solar charge controller. These substantially improve the efficiency and charging capacity of the panels, the life and health of your battery and allow you to combine the panels in series. Most have DC charging inputs so you can still charge off the car on the move.