Easy to solve. Drain everything, no drips remaining.
Now leave all valves open and turn on the kitchen sink water only. Which drain outlet does is come out of? If there are 2 or more valves attached to the same outlet, then close them all off now that water is running. Wait a few seconds and open one valve at a time. When the water starts flowing, you know that valve was for the kitchen sink. Mark it as kitchen sink.
Now, do the same thing with the first bathroom sink. Mark it. Now do it again with the second bathroom sink. Mark it.
That leave only 2 more valves, the toilets. Based on the position of the discharge pipe in relation to the trailer, you should be able to tell easily which one is which toilet. To verify, close all the valves again with all completely empty tanks. Now add water to toilet #1 and open the valve. If it flows water, you know that one is for toilet 1. Mark that one "toilet 1". That leave only 1 remaining, toilet 2. Do the same just to verify. Now mark it toilet 2.
That's is really the ONLY sure way to do it. Every trailer, even with the same model number can be built different depending upon which day of the week it was built. So, the only sure way to know, is to do it by process of elimination as I've just described.
Good luck! Enjoy, and happy camping!
2019 Montana High Country 375FL
2014 Chevy Duramax HD 6.6 - 3500 Diesel Dully Long bed Crew Cab