Well, here's a crash course on tire sizes:
the first # is the width in millimeters, the second pertains to the sidewall (it's a percentage of the first number, therefore for 305/50, one of the sidewalls is 50% of 305mm) and of course, the third number is the wheel size. So, for your size tire they recommended, it's 305mm wide (which is right at 12 inches) and darn close to 32 inches tall. What I always do is convert the first number to inches (divide 305 by 25.4), this will get you the width. Then, divide the second number by 50 and multpily that by the width, this will get you the sidewall height, both of them (Always divide the second number by 50, it just so happens you're looking at a 305/50 tire, don't be confused by this). Finally, add the sidewall height to the wheel.
Sorry for being so long but it's hard to explain in writing. There's a bunch of people that will tell you a bunch of different defintions for these numbers, but my brother is an chemical engineer for Firestone and I've had plenty of arguments with him about this (and he always proved me wrong in this subject) so I believe this to be very correct.
BTW, with what you're hauling, why did you pick the 20's. Seems like a strong enough tire is going to ring your wallet. Make sure you have a good enough rating on the tire before you buy.