In trailer plumbing there are two sources. There would be a check valve at each source to keep the pressure from running out the other source.
The water pump source has a suction line going into the storage tank; the pump itself has a builtin check valve and a pressure switch to set the maximum water pressure.
The city water supply goes first to a regulator to keep the maximum pressure low, in a range the materials can handle and then a check valve.
The way to think about these two supplies is that they are tied to either end of the cold water line.
The electrical in a trailer is actually several systems:
1) trailer towing lights
2) trailer brakes
3) towing charging
4)
12V house supply, disconnect, battery to fuse box to
12V loads
5) Battery charging, and trailer power supply
6) Shore power, power panel to AC loads
7) I will stop here but many folks also have an inverter, solar power options