A 50ft roll of 1/2" pex locally here is $15 (superpex - hot water and UV rated)
Cutter (cheap ones $3) better ones $8-15, but if you can use a utility knife but it is harder to get a nice strait cut.
Crimps are about $.10 each
Crimper locally $24 - Ebay $30 Just make sure to get the crimper style below not the style that is sized for each pipe size. The crimper below will do any size pex with the correct fitting without needing any other tools.
http://www.ebay.com/itm/Pex-Cinch-C...229?pt=LH_DefaultDomain_0&hash=item27bd967315
Fittings vary from $.39 to $2 and shark bites are $5-10 depending on fitting.
The shark bites I would use sparingly, and as a field repair as there are no tools required to use them. Once they are installed the should be as durable as nearly any other connection on a vehicle, very similar to push-lock fittings that are used for DOT air brake lines which I am certain they were based off of. In fact my OBA and garage are both plumbed using it upto 175 PSI and free of leaks to date.
I would use a shark bite as a quick disconnect in your lowest point to act as a drain when required for winter just ensure the pex is supported.
As for the carbon filter I relocated mine on the last truck and if yours is like mine all I had to do was lenghten the hoses and wiring harness to it, just make sure to mount it in the same orientation it is currently in.
Personally I think pex is the best suited for mobile use due to the fact that is the easiest to install and route, fittings are cheap, and it won't colapse like soft hose over time. Only concern would be routing near exhaust, but it not supposed to burn just melt.