Those breakers you’ve used for the 40A one have a very poor reputation unless genuine Blue Sea ones. I’d suggest a midi fuse of the size recommended in the Orion instructions (60A if I remember correctly). It needs to be place as near the starter battery as possible.
Depending how often you plan to use the leisure electrics you may want to consider a switch to turn off the Orion so it’s not topping up the battery every time you start the van. Fine if you are using the fridge etc very often but not great if it’s only occasional use. You can do this with a simple switch in a wire loop on the removable green plug on the Orion (covered in the instructions).
The fuse on tne leisure battery positive needs to be close to the leisure battery. It’s recommended to use a MRBF or Class T rather than a midi/mega and certainly don’t use one of those breakers. Some good detail on the Roamer website about this.
You may want to consider an isolator switch just after this fuse so you can isolate the battery if needed for maintenance or storage.
Depending on how you are using the van you may find the starter battery gets drained while camping due to lots of unlocking, opening doors, interior lights coming on etc. Also if you don’t use the van for 2-3 weeks at a time the battery can drain. An Ablemail AMT12-2 is a great solution to keep the starter battery topped up from the leisure battery. They cost £65, are tiny, very simple to fit and very effective.
Make sure all the cables are correctly sized for power and voltage drop. And the fuses are larger than the max current but less than the cable rating. They go at the power end and protect the cable not the device.
You haven’t shown your loads (fridge, lights etc) - these will need to come off the positive somewhere between the Orion and the leisure battery fuse (or the isolator if you fit one). Each cable to each load will need to be fuse at that point.
