After drilling the 2 holes for sink and shower drains, we dropped the tank and installed the Uniseal Slip Fittings. It helps to bevel the leading edge of the PVC to be inserted into the seal. We used dishwashing soap to lube the seal and pipe.
We wanted to use unions on the PVC drain pipes so that if the tank has to be removed later, we won't have to cut any pipe or deal with the slip fittings again.
The nearest pipe is the shower drain; due to the limited space under the shower, the union has to go though the floor.
In the middle-distance is the vent stack. It only protrudes above the top layer of OSB and doesn't need to go through the floor.
The far pipe is the kitchen sink drain. The union will reside under the sink.
The tank drain apparatus will be added later.
The tank has a 58-gallon capacity, and measures 20" x 13" x 58".