I have a 10 gallon round cooler with slotted copper manifold and my record is a double batch of the Bee Cave IPA. It was 25.5 lbs of grain and the mash fit with about 1/2" to spare at the top. However, I batch sparge and it was impossible to add enough sparge water to collect the required pre-boil runnings at one time. So, I had to sparge more than once. I think this is an advantage of fly sparging.
18.5 lbs of grain is no problem at all for your setup. Use the calculator mentioned in the post above, it's very helpful. Also, if your mashtun has a false bottom, be sure to subtract that volume out if you think it's going to be close.