You don't necessarily need to replace low point valves with metal as I've seen them in both plastic and metal but the ones I've replaced, I did use metal. I'm a fan of SharkBite and that's what I've used in the past for several replacement of low point valves. Many RVers I know have used SharkBites in that application. However, I think that's overkill as they are quite a bit more expensive than you really have to spend.
Example images of plastic:
HERE and
HERE
Example of Sharkbite at Home Depot:
HERE and
HERE
I've used the Sharkbite push-to-connect but as I say, the price is pretty expensive for what you need it for so $16.85 compared to $6.11 for the ones I'll link below.
However, these are the ones that our park model manufacture used for low point drain values and also used the same ones for the toilet and under sink, etc. and would probably be the ones I'd use for an RV if I ever had to replace a set again as they are very easy to use as it's just a quarter turn for on and off and they're easy to grab with just a thumb and forefinger plus they seem very sturdy ...2 for $12.22 currently through Amazon so quite a bit less than a SharkBite ball valve. I've seen several Class C motorhomes and travel trailers using them or something almost identical.
PEX Straight Stop Valve
Linear Mode
