Check for loose/corroded battery terminals.
Then put a meter on the battery and watch the voltage.
The battery should be at about 12.6 volts or more before turning the key to the on position, drop less than a volt when the key is turned on, and drop when cranking but not to under 9.6 volts.
If the battery is under 10 volts when the key is off it's the battery.
If the battery voltage does not drop it's likely a fuse/relay, or mouse chewed wire, bad switch, etc.
If the battery drops when turning the key on it's the battery.
If the battery drops to 9.5 or less when starting it's the battery or starter winding.