For good advice you should have given a bit more detail on your "HHR."
I looked in the first manual I found, a 2008 model. I just waded through to page 245 of the manual. It doesn't specify manual or automatic transmission speed limits, just keep it under 65 mph. I suspect it's a safety thing more than a strict magic speed. Given the variables in outside temperature, I can't see how going 64 mph would be O.K. and 66 mph would cause damage.
I've got to mention though, you're driving a RECREATIONAL VEHICLE. For recreation. What's your hurry? The frontal area and shape of the RV will create wind resistance that increases exponentially as you travel faster. 70 mph may seem a better speed, but I bet your gas goes faster too. Slow down to 60 or so and see if your gas mileage doesn't increase. The time you waste stopping more to fill up will offset the time gained by traveling faster.