All the plumbing in our motorhome is standard 1/2" threaded, PEX or a combination of the two. Some manufacturers may use other fittings, but it doesn't make sense when all the standard stuff is mass produced and should be cheaper.
Not sure which end of the reducer is non standard. The Watts valve should certainly be standard NPT. If it's the filter housing or another fitting leading to the housing, I would replace it with standard while you have everything apart. If your RV plumbing is PEX, that is becoming very widely used even for residential. Most hardware stores will have PEX crimpers they will loan or rent.
You can also use Shark Bite
(or similar) fittings that will fit PEX, PVC and copper. Shark Bite fittings slip on over the pipe and seal with O-rings or rubber seals inside. Shark Bite fittings are also available to adapt straight pipe on one end with NPT on the other. The fittings are expensive, but no tools are required for assembly.