I don't think that boat weighs 3,000 lbs; pontoons are pretty light for their size. I'd say something that size would be in the 2,500-lb range. If you drive by a truck stop with a scale, it may be worth spending $10 to get it weighed so you know for sure.
It's all about wind resistance, and you see just how much of an effect that can have. On the downside, there aren't many ways to mitigate that other than slowing down. As long as your transmission and engine temperatures stay reasonable you shouldn't hurt anything, but you're going to drink a lot of gas pulling that wind sail down the highway.
For comparison, I put my 18' bowrider on a scale a few years ago, and it's right at 3,500 lbs. My towing experience is very different from what you describe; I spend most of my time between 6th and 7th gear on the highway, which keeps my engine around 2,100 RPMs or 2,700 RPMs at 65 MPH. I very rarely tach over 3K unless I'm accelerating.
2019 Tesla Model 3
Former: 2017 Chrysler Pacifica