As mentioned earlier the outdoor shower is the low point drain. Having said that we added 2 shutoff valves behind the panel but for a different reason.
In the fall we drain the system and install antifreeze. If we decide to use the coach during the winter months I shut off the valves to the outside shower with the antifreeze still in place. Then I flush the fresh water system and add water to the tank.
I used 2, 1/2" Flair-it fittings available online or from almost any RV supply store. Depending on where you get them they range in price from $9.95 to $18.95 each. It takes about 10 minutes to install each valve. The only tool required is something to cut the PEX water line. Then you slide the nut over the line, push the line over the valve and hand tighten the nut.
When traveling down the highway at 60 mph in -10* weather I don't have to worry about water freezing in the shower valves and hose. The under floor heater keeps the tanks and internal plumbing warm enough, but doesn't have enough output to keep the uninsulated cabinet warm in extremely low temperatures.
As a side note I haven't seen shut off valves installed on any motorhomes other than a few high end Prevost models. There are many reasons they cost $1,000,000+ rather than the $100,000 to $200,000 for most Winnebago products