Google's your friend. Here's a link to all the specs for your 5er.
Link for your 5erThe LED's are not very accurate, so you can't go by them. After a few times out, you'll know how long you can go before dumping.
Lwiddis was just saying that by being at a Full Hookup Site, it's the best way to be able to tell how long you can go before dumping. Because once you get full, you're able to dump without hooking up and driving to a dump station.
Knowing the capacities help, but first hand practical experience of how YOU use it is best.
Mike