Welcome to the forum its a great place to get your questions answered. If I read your question correctly I think you can find the answer by filling the two gray tanks with clean water to full then one at a time pull the dump valves (ones in back) and see which drains.
With propane, if the regulator is not an auto change over then yes you need to only have one tank at a time open. If the regulator is auto change over like most are then it is recommended that you have both tanks open. The one the valve is pointing to will deliver full propane flow to your rig and when the auto change over takes place there will be a slight reduction in total flow (I have never noticed the reduction in flow with my rig, but that what the propane regulator Mfg says) Let the forum know what you find.
Jake my sidekick (yellow Lab) 10/04 - 05/20
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho