My solution to the cheap broken plastic drain valve is similar to Gaylon's. I bought the same flange he did on eBay <
http://www.ebay.com/itm/181845510888>. It's pricey but well made and the mounting holes are the same as the old valve. When I unscrewed the old valve from the back camper wall I was able to pull out enough of the plastic drain hose to clamp with my little Vise Grips and then install the new flange.
I did get the petcock valve as recommended (see pictures) but have found I like the brass plug better. No worry about breaking it off and water flows out faster than with the petcock. I don't drain the tank often so all in all I like the new setup.
I did the same thing!