Hi Dave,
I have checked on ebay and the Smartcom is an 'intelligent self switching' unit, so you only need 1 wire.
I have a different relay which needs a relay trigger to operate - I use the second wire from the alternator.
The only reason why I have a second wire from the van battery is so that I can put a change over switch and run the pump from the van battery if the leisure battery becomes is flat. We used that on my son's van on Saturday to finish a job off when his battery 'died.'
So, with the unit you have you only need 1 wire from the van battery to the Smartcom unit, but you will need to also use an earth wire from the chassis to the Smartcom. The connections are clearly indicated on the unit. You must use 2 fuses, the first as close to the van battery as possible and the second as close to the leisure battery as possible.
Another thing to remember is that this Intelligent Relay doesn't click in immediately your engine starts running as it waits for the van battery (starter battery) to become fully charged before it then kicks in to charge your leisure (slave/pump) battery. You will hear it click. Please don't be tempted to adjust the adjustment screw to change the kick in time.
Spruce