Thats fine. The most important thing is to ensure you have some decent cable thickness.
There is an ebay supplier, Split-charge-systems that supply them as a kit. If they aren't listed, email him through ebay and he will sort you out. Nice helpful guy.
These relays are bi directional. This means that they will charge your van's battery as well if you leave the leisure battery in the van to 'bench' charge it.
The relay will come with an earth wire to attach to the body. If you want total control over the function of this VSR, then put an inline on/off switch into this line. If you switch the switch off it will stop the vsr from operating at all.