And...
Unecessary...the original Westy split charge relay (in my California) was a double make 5 terminal 30 amp unit, which to be fair was a bit under rated but going from that to one of these (starter) solenoids plus (additional relay)is ridiculous
If you have a fridge then the best 'upgrade' to the standard relay (in a Westy) or any other conversion for that matter, is a Durite or Lucas SRB 630 which was made for the job and has a separate feed for the fridge which it isolates when the engine isn't running. This can use the standard wiring (just needs 4 terminals remaking) Like this:
http://www.autoelectricsupplies.co.uk/product/1027" onclick="window.open(this.href);return false;
Otherwise and if this was a new install I would go for a simple SMART voltage sensing relay between the two batteries like in this link:
http://www.autoelectricsupplies.co.uk/p ... tegory/149" onclick="window.open(this.href);return false;
Otherwise: the cheapest upgrade option is just a heavy duty 70 amp 4 pole (but this doesn't have a separate fridge feed like the dedicated Durite/Lucas but this might not be needed)
http://www.autoelectricsupplies.co.uk/p ... ategory/36" onclick="window.open(this.href);return false;
The important points are to have sufficiently rated wiring (60 amp would be good) and something like a 40 - 50 amp fuse close to the starter battery.
Martin