'18 F150 with 5.5 box tow a 5th wheel ?

They are now making light fivers. Certainly you wouldn't get anything large, but I would look at the smallest one you could find then go from there. Certainly you are limited by capacity. That's why in our entire rv life I have always waited until I had enough pesos to buy at least a 3/4 ton. (Im not ragging on you its experience) We started with a 3/4 k2500 Suburban and towed 2 tt's for 14 years, now we upgraded to a 3/4 ton cc f250 short bed with the 6.2 l gas and tow a lite 37 foot fifth wheel. It works great and is a perfect match. As we only tow around a 1 month a year I wasn't going to waste $ on getting a diesel and we do fine on the hills. This model has a 6 speed. Perhaps consider trading in your TV if you cannot find something that you like and can safetly tow. BTW we have the Pullrite Sliding Hitch and it works perfectly with the short bed.

Happy Trails