The '08 40X has a large 1 1/2" dump valve (there is the small drain in your water bay as well, kinda useless though). It is located inside the bay immediately in front of your curbside rear tires. It is a bit awkward to reach, at the top of the bin on the subfloor.
We have a long shallow fresh water tank, forward of that valve (you'll notice the valve is on a long PVC loop; it connects to the tank and releases the water up near your propane tank).
I had to replace my valve--it's a bugger to work in that small space!
This is the cold water drain in the water bay, beneath the city water input. Note: This is a very old picture. Don't ever leave this type water regulator sideways as shown--it will still work, but I drained all the oil out of the gage. It should be kept horizontal at all times, including if you use it on a park pedestal. (I've eliminated it entirely by installing a Watts house regulator permanently.)