Kingham heating

Just to add to this, there is also a 'T' behind the refrigerator where a branch feeds upwards into the cupboard above - you can see the vent outlet at the rear left side of the cupboard. This further reduces the heat available at the front. I have considered attempting to block this off but it's not easy to get access to remove the 'T' which would need to be via the lower external vent where the feeds behind the refrigerator are boxed in. For now, we along with others are using a small oil filled radiator at the front of the van.

I have the same problem on my 2017 Kingham. I contacted AUtosleepers about it and they took the van in to investigate, they even got the heater company into check it out (or so they say). They told me that they put the van in to a cool room and tested the heating and all was within the specified standards, but when I asked if they could supply me with the readouts from the test and the location of the test points within the van they said that was not available!!! After many arguments about the vent system not being fit for purpose and them saying that the reductions in size and length off the ducting was to blame for the lack of heating at the front and so put up with it and basically * Inappropriate Word * off, I gave up for the sake of my sanity. So I purchased a good Silver screen and 2 snake style draft excluders, one for the bottom of the sliding door and the other for
the vertical gap between the sliding door and the kitchen unit (where all the drafts seem to come from). Last month I was on llangynidr moor, -3 c outside, howling wind and snow, and I had to turn the heating down from my usual set point of 21 to 16, and my feet were not cold when sitting in the passenger seat watching TV. So Removing the heat loss from the windscreen and getting rid of the drafts caused by the bare door metal seems to have done the trick, no thanks to AS

van in for hab/damp check today (not AS but at AS dealer) so had a good opportunity to crawl over a Kingham and have a look at the heating runs...
as said, four vents...two at back, one exiting front of bed box (large bore pipe), one has junction and spits into small bore to washroom and large bore to rear of bed box.
two other large bore pipes heading forward but both reduced to small bore to pass fridge with one spuring off to wardrobe.
other spur and other pipe both head towards side seat box and, in every other installation ive seen, would have exit vents in face of side seat, supplying heat from two pipes (albeit small bore) to lounge area.
however, the two pipes just disappear into a long thin box (similar to the one in the wardrobe) which might have tiny exit vents, but the point is, these vent into the seat box, not into the lounge...
it would be very straight forward to add two vents in the seat face, though the pipe size would need to be returned back up to large  bore first...
the fact that the piperun remains inside the seat box would keep it warm under there, but the majority of heat would pass into the lounge, where required/desired.
a couple of caveats....
first, I would disconnect the pipes at the thin box under the seat, turn on the heating (try electric, gas, mixed, fan on eco and fan on high) to ensure that there is sufficient heat coming through to warrant considering the changes...
secondly, im not advocating that anyone without some basic DIY skills and the necessary confidence/competence should tackle this (albeit straightforward) job.
however, provided the heat is coming through as desired, the Truma pipework is all push fit and the holes required are pretty easy to make...
if in any doubt at all, please consult a competent 'person'.
good luck..