Here you go. Take it to Metalink for a chassis strengthen and longer drawbar for gas, fuel, battery and bicycles. Put a 1-1.2m door in it, take out the bunks or have them fold up, move the fridge and sink to the centre or take out the dinette and put it there.
Put in a bulk head across the roof and down the wall near the door. If the frame and the rest of the van is in good nick you'll have a toy hauler for about $10000 depending on what you want in it and how much you do yourself.
IMO just go for a normal pin back door and use a plank. The drop down doors that work as a ramp are heavy and are always in the way when down. You could make the door with a normal size door inside it so when the bike aren't in it, it becomes a normal van. I would also put in a slide out fridge BBQ under the bed.
Geez, maybe I should go and have a look at it
http://www.gumtree.com.au/s-ad/maroochydore/caravan/1978-millard-caravan-sleeps-5/1036250003