I'm sort of answering my own question. I swear that I thought I'd see a PEX that was rated for gas, but I was just told by someone that PEX is only for water.
If this is true, then I'll have to use copper for my propane line. THis will be an issue, but I guess I can make it work.
I'm building an earth home. Where the propane line comes into the house, is in a 2inch hold in the footing. On the outside of the footing, there's a french drain about a foot from the house, so the propane line needs to immediately bend upward to avoid that 4 inch pipe and get over it and then across the yard to the tank. In addition, the area next to the house here, will be buried. This means that this line needs to be low-maintenance.
On the inside of the footing, the propane line will need to go about 40 feet to a distribution point where it goes to water heater (tankless) and to the kitchen stove. Nothing else in the house will use propane.
This line inside the house wont have any issues as far as things to avoid. I should be able to run it directly across the house. It'll all be below the floor (concrete).
So I'm wondering, since I can't just run (end to end) contiguous copper (because of the bend at the footings, there's going to have to be a weld. Should this weld be on the outside of the house, where it will be buried, or should the weld be on the inside under the slab? Is it going to be pretty low maintenance either way?