I'm a little confused about the terminology in your question.
By "Pop-Up" do you mean a pop-up tent trailer or a Pop-up FGRV like the Hunter Compact-II and Campster
As it turns out I have experience with all three and have owned all three at the same time.
That said, comparing a 13' FGRV, like a Scamp
, with a pop-up tent trailer "of similar weight" is somewhat like comparing apples and hand grenades as all they share is weight
. A 13' Scamp, at 1500+ lbs is a cozy hut for two + 2 kinder if you don't have a bathroom. A 1500 lb pop-up tent trailer can be sleeping space for 6 adults and still have more open floor space than the Scamp had when empty.
With 50%+ less frontal area it's a no brainer that the pop-up will be easier on economy and less prone to blow around in side winds.
About actual towing, it's been my experience that frontal area is the more critical issue. When towing I saw about the following MPG losses when compared to non-towed typical mpg's.
TV = 2009 Honda CRV 4 cyl/auto,
Scamp 13' -40%
, 13' -40%
Hunter Compact II -25%
TV = 2003 Blazer 2 dr V6/5 sp std.
Hunter Compact-II -15%
StarCraft Pop-Up -5%?
The Scamp was the heaviest of the bunch, but the lighter weight
of the similar shape Lil'BigFoot didn't make much/if any difference.
The Hunter, while only slighty lighter than the Lil'Bigfoot, has about 30% less frontal area.
The Pop-up tent trailer has very little exposed frontal area, most being displaced by the TV
As shown, the normally much higher MPG figures for the newer 2009 CRV, took a bigger hit than the Blazer when towing.
Bottom line, the pop-up tent trailer is an entirely different animal than an FGRV and, although will usually result in less reduction in MPG's should be compared for entirely different reasons. For concerns about towing Pop-up tent trailers; <<<popupportal.com>>> is a huge site with about a bazillion members and 100's of posts on this topic re: pop-up towing and MPG losses.
BTW: We never intentionally tow faster than 55 MPH