Hi Patriot_Multisport,
The 4018 has a GVWR of 18,000 lbs. For planning purposes, you can expect 20% of that weight to rest on the hitch/rear axle of the tow vehicle. Add to that the weight of the hitch, bed liner, bed cover, tools, passengers, pets and anything else in the truck and you'll need a truck with a payload spec that matches. I would guess you could easily be in the 4,600 lb payload territory.
A 1 Ton (3500/350)
dual rear wheel truck will put you in the right zone for both payload (vertical load) and 5th wheel towing capacity (horizontal load).
Check the actual specs on the truck you're looking at, before you buy. Depending on how they're equipped, capabilities vary quite a bit.
If you want to get more precise, check out
Fifth Wheel Safe Towing.