Rather than drilling lots of holes, have you thought of opening one of the van doors?
HiDrill a hole in the floor using a small hole saw cutter for steel. Go to a motor factors and look for a rubber grommet that will fit in the hole and allow you to feed the delivery hose through. Make sure the hole is in the correct place and not into a box section or other structural member.Drilling a hole in the side of the van will cost money to repair if you want to sell it on later. Drilling a hole in the floor can always be 'repaired' using a blank grommet. Putting the ply lining back in the van will hide any holes. Drlling a hole in the floor will provide easy access to a distribution manifold under the rear bumper so you can connect your hose reel outside the van and lock it up whilst you are at the back of customers houses.Anyway to fasten the tank in you will need to drill holes in the floor, and if you are going to fit a split charge relay you will need to drill another hole for the supply cable.Spruce