Wait till you have it loaded to normal towing weight after you have loaded it with your equipment etc.before taking another mpg reading.
When we bought our new crusader it was equipped with the BPW ITC control (the equivalent of the Alko system) on the BPW chassis. I had never before had a caravan fitted with it and on our journey home from the dealers we had to travel some miles on country roads and at fist felt that something was wrong as I could feel the caravan constantly braking itself over most undulations and surface irregularities. I then realised that the caravan was bouncing around as it was running light and the system was constantly activating.
After loading all our bits in the van this stopped and have not noticed it since,
I know you were bringing it home on a motorway journey, but if there were any sidewinds or uneven road services, with the van empty could it to engage more than it would normally due to movement on the van.
If you find it still a problem when caravan loaded, I would get your dealer to check the system as apart from the fuel consumption yoiu could overheat the brakes.