Things I have found, some on a car, Some on the motor home.
Bad connections (in my case it was the battery positive due to poor engineering it kept going bad.. Since my degree includes what they did wrong I fixed it when I replaced the battery) I should add most any of the connections Batter + or =, or the chassis end of the Negative cable are prime suspects.. next are the connections out along the Positive line.
The Disconnect switch mentioned above is good too.
I have had bad cables
And if you are just taking this puppy out of storage.. DEAD Batteries.
This is only a partial list
Oh, someone up-thread mentioned the Neutral Park switch.. Yup, had that one happen too (Bit my wife, not me, I put the thing in park and she fired right up).