We have no problems camping in the high 20s and do so often, but I don't want to say it's OK as I don't know what rig you have. We just got back from a two week trip where temps were below freezing (32) every night and dipped to the teens a few times, but daytime temps always were above 40 degrees or so.  
If you have an enclosed underbelly I would think 28/29 degrees for a few hours wouldn't cause hard freezing of your plumbing, but once again I know what our rig is capable of and it does not have an enclosed belly.  Maybe just drain tanks and open low drain points on your rig would be enough, a few more minutes and you could go the full process and have peace of mind.