Newbie Water regulator question

I am the new owner of a trail mini, also new to RVing. When I hook up to city water I know I need a water regulator. The one I have is preset at 45 psi, but I think the manual said 35 psi is the safest. Does any one have any recommendations? I also wanted to know if the hot water heater fills up automatically when you fill the fresh water tank or use city water?
Getting ready for the 1st real trip
Debbie