I haven't seen your valve or your trailer, but a common dump valve is glued to the drain line.
The good news is that there are four screws on the flange so the valve can be disassembled. You could clean it but it is easier to buy a new valve and disassemble it. Reassemble using all the new parts except the inlet side which you can just clean. A little care must be taken with the rubber o ring. Use a little silicone grease for assembly lube. This is a simple repair which can avoid cutting and gluing.
All this assumes that the valve looks like this...