You're not going to like my advice. Don't buy a manual sliding hitch.
If you forget just once in the next __ (5, 10, 20?) years to manually slide your hitch and strike the truck with the trailer, you will wish that you had installed the expensive automatic hitch. I considered the $2500 or so to be pre-paid insurance.
Pulliam Enterprises Super-Glide hitch is the only automatic slider I know of. I recommend you buy one.
Sorry, I told you that you wouldn't like my advice.
2012 New Horizons 36' Travel Trailer, Front Kitchen
2008 Dodge Ram, 6.7 liter Cummins Diesel, 4WD, Quad Cab