Now, that said, and given your tank is relatively small since it’s in a pop-up, you stand a better chance using a repair kit than say, a large container from a 45-foot motorhome. I’d try the Eternabond method first; especially if there is no stress at the crack location. Fiberglass patches will not adhere properly to polyethylene. So I’d suggest either Eternabond tape or one of the commercial two-part epoxy kits if you cannot locate a shop with a plastic welding machine. The last resort is to replace the tank with one with thicker walls.