(Not) Charging caravan battery while towing.

[mir60899]

Hello to everyone out there,
Just came across a bit of an oddity on our 2006 Elddis Knightsbridge (Avante clone) which we purchased earlier this year.
We have used it quite a bit over the time we have had it but never realised until yesterday that the on board battery is not recharged whilst towing. The thing that gave the game away was using the motor mover to put the van on the hitch at our last site.
Now on our previous van, also an Elddis but a 1998 Jetstream, we would arrive home from the same site and on taking the battery home find it was almost fully charged, needing less than one hour to bring it up to fully charged. This time the battery tok some four hours to fully charge.
An inspection of the owners handbook showed no connection to the split charge circuit on the car, although the 12 volt fridge feed was present.
Not believing the book I took the covers off the 12 volt system and using a multimeter proved that this was indeed the case.
So now I have had to design and build a suitable circuit to get round the problem.
Does anyone else have a similar experience? Because if we had been going to a non hook up site we would have arrived with only a partially charged battery.
Very strange!