You dont need a ramp, just put strawbales in front of the back doors, put food on the straw bales, put more food small amounts at intervals inside the van working backwards towards the drivers seat, the pigs will go on to their front legs to reach the first food, climb onto the bales for the second lot, then into the van. Warning, it can be very disconcerting driving long distances with a pig's head stuck between driver and passenger. You may be ok with a quiet GOS however a voluble tamworth is a different proposition.