Caravan Heating - Am I going wrong somewhere?

Three vans ago we had a Compass Rallye van with a Truma Electric/gas blown air heating system.  It was very efficient and the electric system heated the van up with energy to spare.

We then moved on to a Compass Magnum 544 also with a similar blown air system but this struggled to achieve a comfortable temperature and the trunking system hardly provided any heat at all.  I know that this was a larger van but not that much larger!

This year we swapped the Compass for a new Swift Challenger Sport, again with a Truma blown air heating system but this is similar to the Compass in poor efficiency.  I know that switching to gas does produce the required temperatures but why doen't it on the electric side?  Matters are not helped by Swift putting the heating controls in such a position that you cannot read them!  However, I have tried every different combination I can think of including the recommended ones but still no joy.

Can anyone enlighten me further?  Is there any other system that can be installed that would be more efficient?