I put dual 20's in my 19'. They're connected by two 1/2" hoses. One filler, I plugged the other with a rubber boat plug. The vents MUST be completely separate, or the second tank won't fill. It defies the laws of physics, but it doesn't work, don't ask me why. I t'd them together and after a day, the second one hadn't leveled off.
It still takes 15-20 minutes to fill them, as the second one is slower. I don't care, because I fill at home anyway. I travel with both tanks full, and only dump at home. Zero issues, but with a fifth wheel and adequate tow vehicle.
I put deck hatches in them, they could be filled that way, also.
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