A lot depends on your specific RV. If it's set up to handle below freezing weather (as I think a Phoenix Cruiser would be), with the tanks enclosed in the heated envelope of the house, and you keep the interior warm while underway (which with a Phoenix Cruiser you undoubtedly would), there should be no trouble at all with turning on the heat, dewinterizing, and going with full tanks and full use of the plumbing system.
A unit with exposed tanks and pipes that is unheated while underway would be rather a different matter, though even there a few hours of upper 20's would probably still be okay...probably, but not absolutely certainly.