Only you can decide if you want water filtration as it enters the rig, under the sink, or none at all if you wish. Parts can be had at most hardware stores, I have a Monaco and it is 1/2 inch pex which can be picked up at Lowes or Home Depot, they carry pricier shark bite fittings, CW or most well stocked RV part stores will have a variety plastic fittings for any configuration possible.
We eliminated the water bay filter in our Monaco since it is prone to freezing and installed a filter under kitchen sink, don't need filtered water for our showers, toilet, or for doing the dishes. We cook with our filtered kitchen water but still drink bottled water 99% of the time, grew up where water is hard and heavily chlorinated, I guess we've become water snobs over the years.