PEX

I do agree with the "Measure it" recommendation. Or take a SHORT section to the store with you.. and remember

When the Shark Bites
Into Pex Dear
Water leaks, Go away
When the shark Bites
Into pex dear
Connections are here to stay

(yes you can sing that and if you are old enough you will know the tune)

Some rigs have 3/8 some have 1/2 inch.
OP needs to measurements from chart previously posted.

enblethen wrote:
It is 1/2 outside and 3/8 Inside.

For the OP - most PEX I've seen used in RVs these days is 1/2", meaning it has a .485" I.D. with a 5/8" O.D., not the smaller 3/8" size.

That sizing makes it easy to use "Shark Bite" type fittings on PEX. Quick and easy. It seems Home Depot has more SB style fittings in Metal, while Lowe's has a better selection if you want Plastic. Also noticed Amazon has a decent selection. I was looking for Plastic, but I'd imagine they offer metal too.

I've used plastic SB style fittings on PEX in our RV, and metal in our boat. No leaks from either.

Good chart, mobeewan!

My trailer has all 1/2 PEX. Original plus what I added for my plumbing mods.

PEX sizng info

It is 1/2 outside and 3/8 Inside.
PEX tubing

pex piping id & od are identical to CTS , copper tubing sizes .Push on or crimp fittings are the same size. Both adapt to NPS nominal sizing fittings.

Dusty, depends on the make and model. Too general a question like what size tires are on a RV. Measure the outside and go to home depot and compare to see what the inside is.
Bill

1/2".