Assuming you're starting with a 4.5' bed and the 5th wheel hitch centered over the rear axle and the rear axle is centered under the bed, you only have 2.25' (27") ahead of and behind the rear axle. If you move the hitch 20" to the rear, the rearmost part of the hitch would likely be overhanging the rear of the bed. Weight distribution and handling dynamics would be thrown into a cocked hat!!
As Richard said, it's difficult enough to tow a 5th wheel with a full size short bed (6.5') truck - sliding 5th wheel hitches are often required. A 4.5' bed in a less-than-1/2-ton truck really isn't a suitable platform to tow a 5th wheel. Sorry....
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