I've camped in my RV a number of times when the temps got down into the 20's and even sometimes into the teens without any problems, although it's not something I enjoyed and I did worry about freezing water lines. Obviously where the water and holding tanks are and where the lines are run and how well everything is insulated or protected plays a big part in this.
For me a general rule of thumb is it's usually safe unless you're facing a hard freeze which is defined as "four consecutive hours below 25F" by the NWS. Most plants survive light frosts and freezing temps that don't meet this definition.
