Driving 70-80 mph is your choice, but there are a few things to be aware of:
Here's a table that shows total stopping distance for a car traveling at different speeds. Col 1 is mph. Col 2 fps. Col 3 is braking distance. Col 4 is reaction time distance. Col 4 is total stopping distance.
Note that 75 requires about an extra 100 feet to stop compared to 65. And of course these distances are for cars. Your stopping distance with a trailer behind you will be much, much longer.
65
| 95.3 | 202 | 143 | 345 |
70 | 102.7 | 234 | 154 | 388 |
75 | 110 | 268 | 165 | 433 |
80 | 117.3 | 305 | 176 | 481 |
Another thing to consider is the speed rating of the tire. I'm not sure if your Sailuns are rated to 75 mph, but I'll be they're not rated any higher. Some tire engineers argue that the speed ratings should be treated like a tachometer red line for the engine. Crossing the line will lead to damage. In the case of tires, excess speed causes excess flex of the steel, which builds up internal heat faster than the tire can dissipate the heat. The heat buildup damages the internal fibers of the tire eventually leading to catastrophic failure. Damage is cumulative and irreversible.
Yet another thing to consider is the impact forces you're applying to the trailer. When you hit a bump, the faster you're going, the higher the vertical force up and then down. Combine that with the bad road surfaces in many parts of the country. Somewhere down the road you may discover serious problems that you'll want to blame on the manufacturer. I can't speak to the trailer's engineering design point, but I'd be surprised if it includes driving at 80 mph.
Your call.