Jim:
You might try thin electrical wire instead, either from a well-stocked electronics store or scavenged from kaput electronic components. It's a lot cheaper than "scale" aftermarket line, and has the advantage that, depending on the type and size of the wire inside, it will hold a great coil shape for the truck lines. The same quality makes it a great resource for any kind of detailing of engines, cockpits, and so forth.
If you need actual tubing, the wire can usually be pulled out leaving the hollow insulation. It's also available in a variety of colors, though of course it can be painted as well.
Greg