Hey Dan - Dumb questions are OK here. Welcome to the forum.
For starters, the other comments are right that you should definitely turn off the pump when you're connected to city water. It's an either/or thing.
There's something interesting about what you said though. You mentioned that you have a 40 PSI regulator and the pump is also running. I don't know this for certain, but I would take that to mean that your pump puts out more than 40 PSI and is making up the difference between its max and the 40 PSI coming from the city inlet.
(That could be totally off, but it's an interesting puzzler.)
You also bring up another peculiarity of the toy hauler community related to full hookups. Most of the toy hauler owner's I see here and on other forums are primarily boondockers and we feel like bumpkins when we go to an RV park and don't know how everything works. My wife and I took a 10,000+ mile trip last fall and I still felt like an imposter at the end, even after setting up at RV parks for months.
I'm used to seeing my buddies unpack all their gear and level up their rigs by digging holes in the dirt. On the rare trips when we have hookups, it's funny to watch everyone staring down the end of their water hoses and tangling up in their shore power cords like circus bears. It's such a different culture.
