Use 2 short (1 to 1.5 metre) double looped strops but passing one end through the other around suitable point on chassis in a "choke" arrangement, same on other side, then putting the 2 loose ends on the winch hook, giving the V effect as mentioned in another post. Be aware of this though: if the ground you want to pull your van up, slopes away to one side, then the caravan will go this way first before it comes in a straight line to your winch. The pendulem effect I think its called. So ensure there is nothing solid in the way!! By using this 2 strop method if van starts to go off in one direction, and it almost certainly will, the nearer strop will go slack and the further will take the load and tend to straighten the van back to the winch. Were you to use one large V shape, then in this scenreo the hook would slide along the V in the rope/strop, and if it went too far could pull on, and hence transfer, some of the load to weak parts under your van like corner steadies or extreme ends of the chassis rails.
Some people winch cars with the handbrake slightly on for 2 reasons
1 it keeps tension on the winch cable. When you stop winching the casualty could roll a little more and allow slackening of some of the coils on the drum. This then leads to "wrapping" of the cable, slack coils crushed by tight coils, weakening and damaging your cable.
2 Should cable break, the casualty/van won't dissapear at (such) a high rate of knots, you may have some chance of catching up with it and pulling the brake on fully! (Not that I'd fancy getting in front of a runaway caravan!!)
Having said that I am not sure what the effect of pulling a caravan backwards with its parking brake slighty on might have on its auto reverse machanism. I stress again here though, slightly on - enough to provide resistance and partially arrest movement if winch cable should break, not with wheels locked up!
If you've never used a winch some training (perhaps from your local 4x4 club) would be a very good idea. They can be very dangerous things in unexperienced hands!! (winches that is - not 4x4 clubs!) Happy caravanning Andy Palmer.