Take a pocket knife and with the headlight switch on use it to push between the battery cable end and the post even though it does not look of feel loose it will tell you if no contact is made. Do it on both the positive and neg. terminal if possible. If they are designed so this is difficult to do then remove them and brush with metal bristle brush and replace.
There is a coating that can form on snug, but not tight, contacts that will insulate the contact points from each other. Especially if left unused for a time.
As a further test you can check the battery for voltage to be sure it is not discharged and if it is then look at the battery separator circuit for a similar problem and check its solenoid.
Battery not starting the motor and not even able to turn on lights could be two totally different situations.
Hope this helps
Good Luck and keep us posted please. "Q"
1999 Newmar, Mountain Aire 3768, V-10, CAI, Headers.
"Spending our kids inheritance one trip at a time"