I am considering a pop-up truck camper for my Ram 1500 4x4. Is there a considerable difference in gas mileage between a pop-up and a regular truck camper?

Wind resistance is a big factor with truck campers, so you would likely see gas consumption increase with a regular camper versus a pop-up, although I don't have any figures to back this up. My personal experience driving into the wind on the I5 in California was that the wind made a huge difference to the gas mileage; so much so that I just found myself a nice semi to tuck in behind and stayed there until the wind subsided. The speed you are travelling also makes a big difference; at 70 mph my gas consumption was much higher than at 55 mph.

It depends upon how fast you plan to drive. At 55mph with a hard sided cabover on my pickup I would get better gas mileage than with the shell on the same pickup and driving at 70mph (and making my own headwind in the process).

The primary advantages of popup campers is lower center of gravity and more clearance for branches and other possible obstructions. They often weigh under 1500 lbs. as compared to the average hard-side camper that weighs well over 2500 lbs. (there are exceptions but they usually lack a bathroom/shower and holding tanks, etc.).

I prefer the added storage of the cabover camper as I can stow duffels and the like in the sleeping area and have the full use of the "living space" during the day. At night I toss the stuff into the dinette area to use the space for sleeping. I can't do this with a popup camper.