A couple thoughts......
First, when the incoming water is very cold it i worth lowering the pressure and slowing down the flow to give the water more time in the tank.
Second, you might want to consider a heated water hose to help warm the incoming water a bit to give the water tank a better chance to heat the water faster.
You can also try putting some insulation or heat tape around the PEX you can reach but most coaches are not designed to deal with real winter conditions.
This is one thing I like about my tankless on-demand water tank. You have a limitless supply of hot water and there is an adjustment to slow down the water flow when the incoming water is extremely cold to give the water more of a chance to heat.