Why don't you take the fully loaded travel trailer to a scale and get it weighed. You are assuming 1000 lbs of stuff, if you weigh it you will know the true number. It could be more or less than you think. You seem to understand the calculations, now if you remove the hypothetical trailer weight you can make an informed decision. If the numbers work buy the 1/2 ton otherwise you can get the 3/4 ton.
The other thing to consider is your terrain, will you be traveling through the mountains or are most of your trips on flat land? Climbing (engine, transmission, cooling) and descending (braking) are where you will feel the limits of your vehicle. In a more extreme geography, most would prefer the more capable tow vehicle. There's nothing like the feeling of standing on your brakes on a steep down grade.