You can safely zip tie your rope lights to the propane line, it should be 1/2" black pipe so very study and well secured. Two other alternatives:
Go with a zip tie that has a mounting screw hold in the end like this:
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Use a self drilling screw to go through the coroplast under belly and into the frame.
Or, use an adhesive mounting tab like this:
Stick it to the coroplast and thread the zip tie through it. These hold pretty well if you clean off the underbelly attaching point good with some rubbing alcohol.
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Both of these are available in Home Depot or Lowe's in the electrical section.
Although I don't know how the adhesive holds up in wet weather. You sure don't want it to let go when you are going down the road. Not that the rope lights are that valuable but you don't want it getting tangled up in the running gear.