Why PEX? Or, why not PVC, ect?
PVC isn't even good to use in a home. They are far more prone to leak is the main reason. Sealant used doesn't stand the test of time with water.
This isn't to say that people don't use PVC for parts of their water plumbing, but when they do it's normally for drains and not sections of plumbing under pressure. When Water pressure is applied it can expand and contract slightly, enough to break sealant over time.
Considering extra vibrations in a BUS, I highly do not recommend using PVC in that application.
I do not recommend.
PEX - While jbeech has had decent results with PEX. It's no substitute for copper, and will only on average last 1/3 as long as copper.
Most bus conversions statistically don't seem to make it beyond 10-20 years as they get discarded, or sold and then renovated, this is kind of the life expectancy we should be shooting for unless you really want this thing in your life for 60 years.. So PEX is sufficint for a 10-20 year bus. Around that time though, you may begin to have issues with the installed PEX. it is the cheaper solution, but PEX replaced copper due to rising prices of copper piping. This is why plumbers don't use it any longer by default. Not because it is better, but because it's affordable compared to copper piping.
PEX Should absolutely NOT be used in an extreme cold climate in a BUS. They will burst if it is full of water and it ices over expanding. Copper can hold much better in such a climate, but it's not immune either.
A Pro for PEX though is ease of installation. Applying fittings/ends to PEX is easier to do than on copper which is another reason people like using it.
Another pro for copper is it's metal and it will absorb and transfer a lot of the cold along the pipe itself like a heatsink would do.
My conclusion. PEX is okay to use on a bus if you plan to only own the bus 10-20 years. I would never use PEX in my house because I want my piping to last 50+ years minimum. If going to extreme climate, get copper piping for the Bus.