I have yet to see a furnace that was NOT plumbed in copper only! I think that the heat would be an issue for rubber tubing. Even in large MoHo's, they run the majority of the LP line in black pipe and then the last several feet in copper.
If you notice, all wiring in a furnace is done in a special '105*C' heat resistant wire also. The bends are not difficult if you get a tubing bender. They are not expensive and are worth their weight!!! Don't forget the double flares that are now required in the RV industry, no thread sealant and don't over-tighten hose flair fittings.
Your mother is done with her lecture now and you kids can go out and play