Adding underbelly weather protection, is it safe?

No different than a utility line within the confines of your home.
Not sure why the other guy thinks sheet metal is bad. How it’s worse than plywood or any other rigid sheathing on a trailer?
Both of you are worried about non issues. Provided you’re competent enough to do competent work and can recognize dangers (like a bad gas line that no one else who doesn’t rebuilt their camper would ever see).