I would run the cable from the battery and down under the van, using cable ties to secure it to the brake pipes. Drill a hole in the floor near the front, paint it to prevent rust and feed the cable through and connect it up to your split charger relay, then on to your battery. You could silicone seal the hole if you wish later.
If you own a Transit Connect LWB, they come prewired for a power point at the rear offside of the van. The female plug is just above the arch of the rear wheel. They didn't fit a power point to the first Connects but the wiring is the same for the people carrier which had that accessory.