Starter relay. No "clicking" noise when key is in START position??
Other than that, TreeSeeker pretty much covered it. Fully charged SLI battery, not connected to the coach (house) battery, and all connections tight and clean.
I've had (all kinds of) electric motors work fine the last time I used them and refuse to budge the next time. So, there's a very (very!) slim chance it could be the starter itself. Most reliable parts stores can test the starter, if you pull it and bring it in.
Also check the connection between the ignition switch and starter relay.
