You asked, "If I can find the Darco, how do I attach it so it's waterproof"...
You can buy Darco fabric at any mobile home (house trailer) supply store or at most RV service centers that have a parts department. The tape you want to use is called "SCRIM" tape. It's available on Amazon as well as most parts stores above. One link is here:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
To attach it, clean the exposed area well with simple green, then wipe down with alcohol. Let that dry thoroughly. Then spray the bottom of the trailer with contact cement, when tacky, apply the Darco to the area. Use a roller or hand pressure with a towel to push the Darco tight against the existing underside covering. Once that is adhered well, use the SCRIM tape to seal the edges. ALWAYS keep the area clean and dry.
SCRIM tape has a paper backing that you'll need to remove, so only remove a small bit of the paper backing, align the tape, press that small part into place, then slowly pull the backing paper and adhere the tape as you go. It's much easier that way than to try to "wrestle a big piece of sticky stuff into position"...