Get a volt meter. A cheap/free Harbor Freight version is good enough for this test.
Set if for 12volts DC and connect it to the battery and read the voltage. Write it down. Turn on the truck. With the disconnect switch in any position, read the voltage again. If it's higher, you're charging. If it remains the same, you're not. Flip the switch the other way and test again. Same thing. Higher=charging.
But note. That test will only tell you yes or no on whether the truck alternator is supplying juice. It will not tell you how much and whether there is enough to actually charge the battery. But give it a try and go camping. What's the worst that can happen?