Yep. Take it from someone with a very small hard sided TT -- 10 or 11 mpg, depending on which way the wind blows. Pop-ups get much better mileage.
But you have to put a price on the camping experience -- with a hard sided TT, setup is easier. Insulation is far superior (we do a lot of cold weather camping). It is much quieter (campgrounds can be noisy). Some campgrounds do not allow pop ups because of bears. Rain can be a problem for popups.
On the third hand, a pop up is more like real camping than RVing -- you are much more in touch with the natural setting, rather than being isolated inside a big plastic box. I am ok with my plastic box because we are outside most of the time, anyway.
So you have to figure out what a popup will save you in gas and then put a value on a hard sided TT. Let's say you take three trips a year of a thousand miles each way, or six thousand miles of towing. With a TT at 10 mpg, that's 600 gallons, at $4 per, or $2400. With a pop up, you might get 15 mpg. That's 400 gallons, or $1600. So you have spent an extra $800 on gas.
The next question is how many nights of camping does that represent. If you camp 40 nights a year, that is an extra $20 per night. Not trivial, but for me, it would be worth it. If I had a pop up, I probably would not camp in the snow and the rain as much as I now do.