My guess it had nothing to do with your capacity! Most likely the set-up on your first vehicle was not correct. When you switched vehicles the set-up changed and your towing experience with it!
I too believe in the 80% rule. I have towed at max capacity and around the 80% mark - I'll take the 80% experience!
Some people are comfortable towing at max capacity while others are not! I personally believe that my driving experience while towing should not be much different then when I am not towing!
You are on vacation - why be stressed out because you are towing?
Happy Camping & travel Safe!