My car is wired up with 2 hot wires on relays, one for the fridge and one for the battery feed, I had them wired in to 8 and 9 pins (2 of the lager 5) and when I got my van switched in the plug 2 to 8 so it was not using the reverse signal pin.
That was a good move, I have seen lots of melted smaller pins over the years. I did the same with mine.
Anyway, when testing the other night, I disconnected the battery but when plugged into the car I had full 12v power in the van so I am assuming that jayco has wired up my battery feed to the aux in the setec
That is the most likely hookup if it was done to jayco's normal. It could still be connected direct to the battery though. If you have a test light you can test the pins on the van trailer plug to see if any are live when not plugged in to the car. the setec will block power back to the plug but if connected directly to the battery you will see power at one of the pins. Make sure your van battery and battery switch are connected and turned on for the test. (no 240v at the setec though)
so my question is when installing my d250 do I disconnect the aux from the setec and connect that to the d250 or do I leave it running through the setec?
Here are a few of your options......
This is how your hookup will look if you leave it hooked up through the setec. It will work fine but you will get a small voltage drop through the blocking diode inside the setec. The setec will block power returning to your vehicle (your car relay will already do this as will the ctek) and ensure there are no live pins at the van trailer plug.
This is the setup for connecting directly to the battery. this will bypass the setec diode and give maximum available charge to the battery. You will no longer have the blocking effect of the setec but the d250s and relays will take care of that so this would be a good choice in your circumstances.
Another way would be to combine your two wires to maximise the power available to the ctek unit. I would only consider this if you don't have a smart alternator fitted to your vehicle though as the fridge draw might cause enough voltage drop to disconnect the ds250.
If it was my van i would go directly to the battery (the second drawing). You must still be aware of the limitations of your wiring. as they are going through the trailer plug they are probably only 6mm so will be hard pressed to supply enough power for the ctek to run at it's maximum before you get caught out by voltage drop. Under most circumstances this will not be a problem and you will still have the solar to fall back on. If you have a smart alternator and find you are getting less than 13.1v to the ctek you may need to do cteks rather kludgy bypass as shown bellow.
Also with this setup is it ok when on 240 to leave the battery switch on so it is getting charged from the setec as well as the solar through the d250 or is that not advised?
Yes it will be fine to leave it all hooked up. The only thing you must ensure is that your charge rate does not exceed 14.8v as this is the safe limit for the setec unit. the d250 will not exceed that.