With the Mega Cab I think I'd err on the side of caution and get a manual slide hitch. You may never need it, but the one time you do, you'll be glad you spend a couple hundred extra bucks to save you from a 1K or more repair bill on the truck.
The bed of the Mega Cab I believe is actually a couple inches shorter than the normal short bed, and it's that inch or two that could get you in trouble.
I've got a shortbed Ram 4x4 and with my Sundance, I don't need the slider, but I'm glad I bought it, as I need it to back my trailer down the hill in my driveway. I've got roughly 8 inches of clearance over the bed rails sitting on the level, but the rear corner of the bed will just touch the bottom of the camper if I don't slide the hitch before backing down the hill to park mine.
*EDIT*.. I just realized that all the shortbed Rams now have the 76 inch bed like the Mega Cab.. mine is old enough to still have the full 6 1/2 foot bed. Without doing some research into the wheelbases and whether those 2 inches came off just the rear section of the bed or not so that the correct hitch placement can be attained in relation to the axle for the weight transfer, I think a slider hitch would be a wise choice in any case.
Yes, the manual sliders make a little more noise with the clunking when you take off or brake, but it's a small price to pay for the piece of mind you get knowing you can get yourself out of nearly any situation by having the slider.
One of my favorite campgrounds has a rather short incline to pull up to get out onto the road, and that was the main reason I bought the slider just in case... don't need it at the campground so I found out, but I never thought about the driveway here at home and that's where I ended up needing it... go figure.