From my picture, I'll describe what the 4 different wires, sheathed in black split wire loom tubing, are. Moving clockwise and starting in the upper left corner, the first split wire tubing is for the wire (I think either 2 or 4ga) coming from the starter batteries and connects to the red-capped jump-start post. Going clockwise, the second split wire tubing goes from the jump post, through the floor of the van, and connects to one terminals of the battery isolator. My isolator is the SurePower 1315-200. The third and fourth split wire tubings in the background are for the negative and positive leads coming from the house batteries. My house batteries are located in front of the SMB 'basement' storage box and behind the rear axle. The positive lead from the house batteries goes up through the van floor and connects to the second terminal on the isolator. The negative lead coming from the house batteries originally was grounded to the body via a bolt on the underside of the right rear wheel well. Last year I eliminated this grounding point on the underside of the wheel well and instead ran the negative lead up through the floor of the van and connected it to a negative bus bar terminal block located under the rear bench seat and closer to the other house 12V equipment.
Looking again at your pic, because the red wire going from the jump post to the mystery solenoid is not sheathed in the black split wire tubing (as would have been done by SMB), my guess is that your mystery solenoid was added after the fact and not not part of the original SMB build.
Also, in your pic, can you confirm that your wire labeled 'To Bat isolator' carries only current from the starting batteries and not the house batteries? If it only carries current from your starting batteries, it appears the easy fix would be to disconnect the 'To Bat isolator' wire at the mystery solenoid and reconnect it to the jump post (it looks like it should be long enough). At the same time remove and eliminate the red, unsheathed wire altogether at both the jump post and the mystery solenoid. Then, using shrink tubing and/or elec tape, cap off the the white control wire and stow it out of the elements under there somewhere. Since that white wire is probably tied in to the van's run circuit it may save you time in the future having to pull a wire with 12v ignition-on power (such as for a DC-DC charger, rear camera, etc). You can then remove the mystery solenoid altogether. Lastly, you'll want to recheck your connections on and operation of your isolator.
2006 E350 SMB RB50 6.0 PSD