First I'll say that each rig is slightly different and the freeze risk points vary depending on size, design, layout and insul. If your rig is sitting unattended in exterior storage, then its obvious you should winterize if below freezing temps are normal.
However some of us use our RV's all winter and they often sit in 20 F temps with no damage, with a few common sense precautions.
I would ask your son to run an extension cord out to the rig and put a 60 watt trouble light bulb inside the water bay, that's going to be the first area to freeze. A small space heater inside the rig set at 40-50 F would be extra insurance.
Unless it gets much colder that would relieve my worries at 27/28. Nothing larger than the smallest lines, hoses will begin to freeze at those temps. Water may begin to get slushy after several hours below 30, but it takes a lot more to freeze solid enough and expand enough to bust a pipe or hose. Some might be surprised that even their RV antifreeze actually freezes partially into a slushy snow cone consistency when exposed to low temps for long durations.
All 3 of my RV's sit un-drained, un-winterized here at 4000 ft with frequent lows in the 20's, using some minimal precautions. Like your area, I'm usually up in the 60's during the afternoon, so it takes most of the night to drop below 30.