Have you look under the trailer for a low point drain all by its self. On all of my trailers there have been a single low point drain with either a shut off valve or cap that needed to be opened or removed; towards the rear of the trailer.
Another thing that will work is open up a low point drain that you can find and turn your water pump on. Keep all faucets closed and just the low point drain open; this will pump your water out.
Let us know if you find the fresh water tank drain.
2016 34RL CC; 2008 Ram Mega Cab 2500HD, 6.7L, 68RFE 6 speed, 4X4, Smarty S67