buy a split relay, usually have 4 wires..

+ve in, -ve in, and 2 +ve outs.

 +ve in goes on positive on van battery, -ve in goes on negative on van battery, and u only use one of the +ve outs and run that to the positive terminal on the leisure battery in the bk of your van. Also you will need to buy an earth strap to connect to the -ve terminal on your leisure battery and then the other end of the strap goes to a cleaned up bolt in bk of van somewhere to earth the battery.

My dad was a self employes mechanic for 40 years and he got me a auto-electrician to fit mine .Did a real good job and advised me on the best relay to get as their is a few cheap and nasty ones on ebay .
Cost me £40 , plus fitting , but once done , should always be done .
Working well .
Used some top quality wiring with extra insulation , and connections .
All professionally fitted and very tidy .
tested vehicle battery and van alternator output .
Happy bunny as it does save time , charging up at night , saving on house electric , pay for itself in 10 years 

Runs 2 pumps all day long.

yes its there job