Forget that 'magical published MAX TOW Rating'......it is a fantasy number based on concept only achievable in the 'on paper' world.
It is NOT a real world number.
"My truck is rated for 14K. Tow chart shows max tow of blah, blah, blah"
Horse pucky!!!!
Long before that magical MAX Tow number is reached......GVWR, RAWR & Rear Tire Max Load ratings will be reached or exceeded.
Those mfg. MAX TOW numbers come with 'fine' print
150# Driver, 20# cargo base models etc.
Trim level (bells/whistles), options, add-ons, additional passengers etc. etc. ALL end up lowering the real tow number.
Best thing one can do is WEIGH your tow vehicle----camp ready (Everybody, everything in truck cab/bed).
Then you have a real number to start with.
Subtract that real number from your tow vehicles GVWR. That number (Available payload) will tell you 'how much weight' can be added to two vehicle before exceeding ratings (whichever ratings you choose----GVWR, RAWR, Rear Tire Load or NONE)
As for which 5th wheel.
Since you can't go get it weighed before purchasing.......
Use the trailers GVWR. Take 20% of that number...now you have a close 'guesstimate' of what the 'wet pin' could weigh.
Do you have enough 'Available Payload' to handle that 'wet pin'
YES-----NO
At least you have a better idea then 'dry weight plus 1400#' give or take, or TOW Chart shows 13,600# or etc. etc. etc.