The pump works best if upright (inlet at the bottom, outlet at the top). It works almost as well at 45 deg to upright, not very well at all at 90 degrees. The height of the pump relative to the bowl determines how full the bowl is to start. Anything in the line up to the pump eventually drains back to the bowl, since the one-way valves in the pump are not perfect. One of mine is just above the bowl (inlet even with the seat) another is about 8 inches higher. The lower one works better in my estimation. The height of the loop for the inlet water determines how long you pump to get flush water, and how hard the pump works as it must overcome the head in that line. Of course you want that loop above the waterline, but if you can manage it, only 12 - 18" above the seat. I learned these things because I installed two, the forward one worked great and the aft one not so good. After identifying and correcting these problems, the aft one works well too.

I plumbed the intake water by T'ing off the sink drains at a point that was about 5-6" below the water line but still above the seacock. That eliminated an intake, and allows closing the seacock and using fresh water from the sink to flush the toilet. I flush fresh water through it whenever it will be left for a few days or before long term storage, and you can even use it on the hard (till the holding tank is full!). On the aft head, I plumbed in two Y valves so that the same pump can be used to pump the holding tank overboard. It takes some space to get all the plumbing in (and that was the reason the aft head pump started out not upright). It works well, but on the forward head with not enough room for the plumbing I just put in another Henderson pump to empty the holding tank, I like that better. It has a Y on the intake to use as a bilge pump if ever needed.

Picture is of aft head plumbing prior to redoing with the pump upright.