300mm - 400mm (12" - 16") is a lot for stacking boards to support caravan steadies, very likely to be unstable unless nailedscrewed together.
Can I suggest making wooden stools or using tree trunk cut offs (free or very cheap from your local tree surgeon).
Another option is http://www.screwfix.com/p/folding-stool-polypropylene-300mm/5076P?kpid=5076P&gclid=EAIaIQobChMI-93Cw-qL1gIVErcbCh0UawaKEAkYASABEgJMb_D_BwE&gclsrc="aw.ds&dclid=CIGSttXqi9YCFQmn7Qod4p0Cqw" with some timber packers to meet final level.
That is quite an incline you park on, so a stool would need packers underneath to level the stool.
The safe working load of the polypropylene stool is sufficient to prop an unloaded caravan, I wouldn't advise inviting a bunch of friends round for a rave in the van though.
