Shut off your water pump and open all your faucets. That way, freezing pipes will not burst as easily. Additionally, 28 degrees isn't so cold as to freeze the tanks unless it is sustained for a long time.
I had a pickup slid-in camper with PEX plumbing freeze in sub-zero weather and nothing broke. As bigben says, that type of tubing is more forgiving.
If your really concerned and can't supply some other heat, consider winterizing your rig.
2015 Wildcat 282 RKX
2011 F350 Lariat, 6.7 L. Diesel, 8 ft., SRW, Reese R20