I always carry some 3x2 about a foot long. I put this in front of the flat tyre & pull the van up onto it. This gives enough ground clearance to get my Trolley Jack under the AXLE of the caravan. I also carry a short length of steel tube to put over the end of my wheel spanner in case the nuts are too tight.
I'd also advise you to put safety first & if need be, do as I did once & drive a few hundred yards to a lay by to change the wheel.
You'll find the trolley jack very handy on site for levelling the van from side to side & the wood handy to put under the low wheel to keep it level. I carry several pieces of wood of various thickness for this purpose. Some worry about the weight of the trolley jack but I'd rather have the jack than the shopping. A pack of lager won't jack the van up.
I strongly suggest you consider fitting TYRON safety bands in your tyres, they're £65-£75 a wheel but well worth it. They do the job they claim to do very well.