The tank on my trailer is probably far from a "standard" installation, but may be helpful. The service fitting to withdraw water comes directly out the bottom so no water can be trapped in the bottom when you drain it. I don't remember if it 1/2" or "3/4" female pipe, but it doesn't really matter, all I did was use a fitting to adapt from the threaded fitting down to 1/2" pvc anyway. I suppose it is easier to reduce it down than have it too small, so err on the big side. I used the schedule 40 pvc pipe, which they tell me you're really not supposed to use for drinking water, as I figured it is heavier and would be less likely to crack and we really only use the water for the sink and toilet anyway.
The fill is just a hose barb the same size as the hose barb on the fill door like what Don suggests, I have the same setup. I would suggest buying your fill door first, and match that fitting to the one you are going to have installed in the tank.
The the city water connection on the fill door just plumbs into your water system downstream of the demand pump, no big deal there. I would put the city water connection in while you are doing the system, whether you think you will use it right now or not. I didn't the first time, then when we decided to stay in campgrounds from time to time I had to go back and buy a new filler door and redo everything. Way easier to do it right up front.
And there is a smaller hose barb built into the tank along side the filler, again the same size as the hose barb for the overflow on the filler door. Just match that up as well. The overflow also serves as a vent.
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