Ok Ants I will list some of your options bellow. Please keep in mind that without having seen your setup and not being able to take power Measurements etc it does make it a bit hard.
From what we have already discussed it seems that the d250 may be struggling to keep up with your power requirements. This is not due to any fault in the d250 it is simply not large enough to power all your needs as it is currently connected.
So it seems you will need to....
A: Reduce the power draw on the D250 by re-confuring your wiring or
B: Fit a dc-dc charger that can supply enough for your requirements.
I know B is probably not an option you really want to consider but it probably would deliver the simplest and most efficient solution. This would supply the boosted Voltage of your dc-dc unit to all areas of your auxiliary wiring and still have enough reserve to allow for charging. Of course 30 to 40 amp dc-dc chargers don't come cheap and if the existing wiring that is currently supplying your d250 is too small that may need to be changed as well. Plus you would have a D250 you no longer need. Still, others who are starting from scratch might want to consider this.
As for solution A you have already indicated that you want to be able to run your engel while the vehicle is off so it will have to stay where it is (not a lot to be saved there anyway). So that leaves us with the van fridge (which will be a substantial saving).
The best way do do that would be to move the van fridge to the input side of the D250 as I said in an earlier post, this would give your D250 up to 15A more to play with.
Unfortunately at the moment you only have the one feed going to your van so this would mean your van battery would also be connected there as well. While the 6b&s cable you have run will be ok for the job your van battery would be missing the benefit of the higher voltages available from your D250S.
Still it would be a fairly simple mod to do so may be worth trying first and see if you are happy with the results.
Because the van battery cannot feed back through the setec the van fridge will have to get it's power from the vehicle. In this instance that will be good because if you start your trip with the van battery fully charged it will still be that way when you arrive at your destination. It will however mean that if you flatten your van battery it will take longer to re-charge while driving. As you have access to solar this may not be an issue anyway.
A better solution might be to run a new wire all the way from the front of the vehicle for the van fridge. then you can leave the van battery connected to the D250 and benefit from the extra voltage available. This would involve some extra wiring and maybe a 12 pin trailer plug (you will need to use the heavy pins for 15A) or a second anderson, you would also need to separate the fridge and setec wiring on the van.
I would still advise you get a sparky to have a bit of a look before you make too many changes as there may be some other reason for your problems.
As to your drawing there are a few areas you may want to look at.
The 80A breaker is a real concern on your 6b&s line. It is far too big for my liking. A fuse size needs to reflect on the things it is meant to protect. In this case if you are using a small Anderson plug they are only rated at max 50A so it could be in trouble if a fault does occur. Also If you look at the installation sheets for the setec you will see that they require the Aux in circuit to be protected by a 20A fuse for the st20 or a 30A fuse for the st35. As the setec provides no means to regulate the current if you had a very flat or shorted van battery you could get a very large draw through your setec. I am sure the fridge will have similar requirements of a suitable fuse to be fitted.
Also be aware that Ctek only recommend a 30A breaker for use with the D250 and you show 40A in your drawing. Probably be ok but with it's max output of only 20A you are well over.
The other thing to take note of is that the van fridge is always connected to your vehicle aux battery. While some people are ok with this you need to remember to disconnect your anderson plug if you are going to be parked for an extended period. Otherwise you will have both fridges drawing from it, and they will bring it down fairly quickly.
Wow that's a bit to take in
I have tried to explain it all as best I can but If you need anything clarified.... just ask.
