Does this mean that since their OEM fifth wheel hitch is rated at 18k this is the limit?
The limit is on the hitch rating and yes it can be a legal issue.
Gooseneck rating is 20700 seeming to mean they can tow more. B&W makes a hitch meant for the OEM puck system that is rated at 20k. If I use that am I legal to tow a 20k trailer?
Its not illegal to tow over the mfg suggested tow rating. However other legal issues are at play on a one ton SRW truck towing a 20k 5th wheel trailer.
First off a 20k 5th wheel rv trailer may have 4000-4500 lb of hitch weight. These trucks rear axle has a 7000-7230 RAWR and can weigh in the 3200-3400 lb range. Now add 4000 lb hitch load....200 lb hitch.....more lbs for people and gear in the truck. You've severally over loaded the trucks 7000-7230 rawr. RAWR is the lessor of the OEM wheels....tires....rear spring pack.
The truck will need some heavy duty mods to carry a load from a 20000 lb 5th wheel rv trailer.
Tow rating aren't just for rv trailers but all types. A 20k tri axle GN flat deck trailer or a stock trailer can be loaded so as not to overload the trucks RAWR.
Short answer is a one ton SRW won't be legal to tow a 20000 lb rv trailer.
Insurance ?? As others say they will pay just like drunk drivers....speeders....crack users....dumb azz pot smokers or old folks or dumb blonds. But may drop you after you receive that payment.