There is basically no difference between the city water and the "Tanked" water systems
Depending on the RV, water comes in the city water inlet, and (Optionally) goes to a diverter valve called a "Quick Fill" valve by some, this valve sends water either INTO the on-board tank or one into the RV.. From there it goes to a "T" fitting. and on to the rest of the RV.
Water from the on-board tank goes first to the water pump, then to that very same "T" fitting, and on to the rest of the RV
NOTE there may be other "T" fittings scattered either side of that one but for this discussion that is the only one that counts.
The water heater thus gets filled by the very same pipe and valve no matter if you are using "Tanked" or "City" water. Tis all the same to the water heater.
Home is where I park it!