You could probably fix this problem yourself. Here is what I would do:
-Most tank drains exit facing the front or rear of the trailer. They then make a 90 to go out to the side. If the pipe exiting the tank faces the back of the trailer, lower the tongue to the ground. This will keep most of the contents in the tank. If it drains to the front, then raise the tongue as high as you can. If it exits directly to one side, jack up that side. Do whatever you can to keep as much of the contents in the tank as possible.
-The valve is held in place by 4 bolts and nuts. It is very easy to remove. I would put as large of a container under the valve as I could to catch anything that comes out. carefully remove all the nuts/bolts. Slide the valve out (staying as far from it as you can) and let it drain.
-Slide a new valve in place and secure.
I know it is a little gross, but if some "stuff" gets on the ground, I would just put some water/bleach on it and then hose it down. I don't think that fixing this is as bad as you may imagine.
2015 Jay Flight 26 BHS Elite with 320 watts of Solar from AM Solar
2016 Chevy Silverado 2500 Duramax