It's "generally accepted" that you shouldn't run your converter without a battery, as the battery acts as a "filter" to smooth out the 12VDC (which is made from alternating current) and a "cushion" for sudden load changes, like when you turn something on or off.
That being said, I just read a WYFCO installation and operating manual and it didn't say anything about running it without the battery.
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