I'm not sure anyone makes a sliding hitch for the 5' 7" Ram box. Most I've seen were for the 6' 4" box. I would first check with them.
I see in Rams body builder guide your truck is rated to tow 6100 lbs up to 6750 lb trailer depending on your truck configuration.
With anemic 6xxx GVWR and small 3800-3900 RAWR numbers the 5er will need to be in the 5k dry gross weight range.
Payloads run from 940 lbs to 1600 lbs depending on the trucks configuration. You didn't give us much tech info from your truck to go on so answers will be general numbers.
You didn't tell us which Prowler 5th wheel trailer but they start in the 9k GVWR range and go up from there. Even the smallest looks like too much pin weight and too much gross weight for your truck. Gear ratio changes will help the truck pull more weight but won't help your truck carry more weight.
