The answer to the original question, how do you increase capacity of a tow vehicle, is you don't you purchase the vehicle you need for the job or you purchase a trailer withing the spec of the vehicle that will tow it.
Secondly, it was said "You have now learned why 3/4 ton trucks do not work well for 5th wheels"
The correct version of this is they do not work well with 5th wheels that they are not rated to handle. My 3/4 ton is well with specs when I'm pulling my 5th wheel and does on heck of a job doing so. The key here, is I didn't rush out and get a 40' with 5 slides and a fireplace, just because I had a desiel engine. There are 5th wheels on the market today that are light enough to be withing spec and easily towed by a 1/2 ton.
To make it all work, and work correctly and safely, is before you make a purchase you learn what all the numbers mean and you make your choices within those numbers.