I've been using the Rubbermaid 10 gal. "Home Depot" model for over a year, and no complaints. Works as advertised, and I just went the easy way for conversion and ordered a Bargain Fittings kit.
Yes, the Rubbermaid cooler's interior lining will warp; mine does. It's only logical. They are designed as coolers -hence the name- and not "heaters." When 170F preheat water is dumped into the cooler, the plastic lining expands, and warps. I can't see how that affects its performance as an MLT.