Trucks are too tall for fifth wheel

A little off topic but I know that changing a rear end will get you more pulling power but a drop in miles per gallon?. SO with that, how much was the cost to do it and how do you determine how much you gained pulling wise and how much you lose in miles per gallon? Not sure it's worth it IMHO. I tried to figure this out by just going to my years truck build sheet and looking inside the glove compartment which gave me what gears it has. I then went to see what other ratios was offered for that year and was able to see what their tow ratings were for each gear set, nothing about mileage though, as I figured. But am I on the right track as to figuring out the gain on the tow capacity if I changed out the gears? I learned it's not much for mine. One more question; if it is a 4x4 wouldn't you have to change the front differential also?...mmm seems like you would have to. Again sorry for the off topic question.