Should be a plug or a small valve on the bottom of it. You'll have to crawl under the TT and look for it. My Roo 19 had a plug and I bought a valve for it. My Mini Lite had a valve, but I piped it out where I could get to it (see pic).
If you have the plug, you can put one of these in it's place:
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10