New Guy Needs Help!

op asks wrote:
I have a 2007 F-150 5.4 Triton engine with 5 1/2 ft bed. My owner's manual says I can tow maximum trailer weight of 8,500 lbs. My understanding is that with a 5th wheel I can tow more because a lot of the weight is on the truck's rear axle. First question is, is this true?

Not true.
The pin weight from a 8500 lb 5th wheel can be 1700 lbs or 20 percent of that 8500 lbs. Too much for a 3800-3900 RAWR. A TT may have 10 percent or 850 lbs of hitch weight

Third, Can I get a 5th wheel hitch put in a 5 1/2 ft bed, some people say there is a "slider" hitch and others have said that I still would require a 6 1/2 ft bed to do this and a 5 1/2 ft bed I'm limited to a pull trailer. A little help from you pros would sure be appreciated!

Best bet for advise on current availability of a sliding hitch for your particular truck is contact some of the hitch makers.

One last thing my 5.4 Triton motor has also had after market modification by Roush, any idea how much more I can safely pull?

Trucks carry weigh determined by its axle/tire load ratings. Size of the engine doesn't help in this department.
Your biggest issue is those small 3800-3900 lb RAWR/P tires and wheels/spring packs on a std duty F150. They limit the truck to around 900-1100 lb wet pin weight which means your looking at around a 6000 lb wet weight small 5th wheel.
Now if your F150 has the 8200 HD package with those bigger 4800 RAWR/LT tires and wheels then the 8500 lb trailer will work fine.

Tow ratings are for all trailer types and not just RVs so there are some types of 8500 lb trailers your truck can handle its hitch load. Stock trailers come to mind as the trailer can be loaded with the big stuff over the trailers axles and small stuff closer to the hitch