Your drain valve is likely connected to a short piece of hose that’s then connected to the low point drain on the FW tank. That hose may have a kink in it. You’ll have to access the hose a see if that’s the problem. On my Rockwood, the drain valve is mounted in a cover that drops down from the underbelly. It’s easy to remove with a few screws.
2019 Rockwood Signature 8290BS, Reese Revolution pinbox and B&W Patriot 18K hitch
2016 Ford F-250 XLT SuperCrew, 6.2L, 4x4, 6'9" bed, 3.73, w/ snow plow & camper pkgs.
2011 Rockwood Signature 8293SS (2015 - 2018)
2010 Rockwood Roo 23SS (2012 - 2014)