Unless your unit is different you should have two relays, not just one.
The first is the aux start relay, it is almost always located in or very close to the house battery.
The second relay, the charge relay, is almost always under the hood, usually slightly hidden by the chassis battery or tucked in front between battery and grill.
As was mentioned above though, if the house batteries are dead you won't hear the aux relay click and it may have shot the charge relay or blown a fuse.
First thing I'd suggest is to give the house batteries a checking over.
