if it is gray tubing, it is polybutelene (PB). Good stuff except for the crimped connections to ABS fittings. You really don't need to replumb to replace it. Just watch for any fittings falling apart, especially in high chlorine environments (as in heavy water disinfectant).
If you see any corrosion on the fittings - most noticeable on the crimp rings - then the easiest thing to do is to replace the fitting. The Quest fittings available at most corner hardware stores are easy to use and work well.
If you do any replacements, use PEX, not copper. It is the modern replacement for the PB gray tubing. I would also suggest using some of the many easy to use fittings for this tubing rather than trying to go the crimped route. Quest, Flair-It and others work well and are much much easier to install in a trailer than crimped fittings or soldering copper.
If you replace any plumbing, you should also be able to eliminate some fittings. I think somebody went fitting happy with a template at the factory on my B-Van so they could neatly lay the pipe around nice corners. With PB or PEX you can bend it in a radius as small as 10 times the pipe diameter (5" turn radius for half inch tubing). PEX is measured like copper tubing by OD. 3/8 inch is usually sufficient for most RV needs.
Also look for the fittings that go to half inch IPT in order to facilitate connection to faucets and appliances. The standard Quest fittings for 3/8 go to this size and you can also find some fittings for 1/2 inch tubing that also adapt to half inch IPT.
You can cut the tubing with any ol' knife but a ~$15 tubing cutter does a lot nicer job a lot easier (and safer on the fingers!)