Never tried to replace one, but I think they are threaded and likely the drain cock broke right at the edge of the tank -- You will have to put something inside the old part and turn it, something that will bite deeply enough to get a grip but not deep enough to ruin the threads on the tank.
I have found that if something won't unscrew easily it often helps to tighten it and then reverse, but that might work better in metal than nylon/plastic.
You can get a replacement drain cock at an RV place or even at a well-stocked hardware store or maybe even from Scamp
(It occurs to me that Scamp
likely breaks a few during new trailer assembly, so a call to the parts desk [not sales desk] might reveal what tool they use).
Multi-thread drain cock
I would use a small bit of plumber's Teflon tape on the threads of the new part.
If all else fails, a new tank from Scamp can be installed.
Water Tank 38.00
Water Tank with fittings 42.00