You are correct in assuming they allow you to "isolate" each slide. I have a Sanibel with a similar setup. The valves on mine were marked I, II and III, but nowhere was I told what number went with what slide. Through a process of elimination, I was able to determine what valve went with what slide, made appropriate labels and stuck them on the valve body so that now I can open each slide individually if I wanted. You say you closed 2 valves and the front room started closing, so you now know the valve left open controls the front room. Label it, close it, open another valve and see what happens. After 3 tries you have your answer.
2012 F350 Lariat Supercab DRW LWB Diesel
Reese Elite 18K - Trailair Triglide
2014 Sanibel 3500 - everything but the generator!