On my 31sl the galley and shower are the same tank as the other tank is almost always empty. The other tank is the bathroom sink and I guess if I had a washer that would be for it. Otherwise the second tank is for toothbrushing and shaving.
You can put on a shutoff valve at the end of the sewer pipe and leave both valves open to utilize both tanks equally.
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2014 31SL: 2014 F350 6.7 litre crew cab:
Steve & Gloria