Agree with rohrmann's post above. But, I'd do it a little different.
Drain all tanks completely then close all the dump valves. Turn on a faucet (say, the bathroom sink) and let it run for a couple minutes. Pull each valve and see which one drain. Mark that valve. Now repeat with a different faucet, say the kitchen. It would be good to test all the valves again, because you just never know how things "could" be plumbed. Bathroom sink and shower could run into the same tank with the toilet, and then, it might not. So check each water faucet, toilet, shower. Then you'll know for sure, because you sure can not trust any labels on stuck on the trailer.
Who you are right now is a sum total off all you use to be.
2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually