Me tinks if the main concern about how you want to travel is fuel mileage over comfort, you might want to rethink your travels. I do not say this to be mean spirited. Maybe the thought would be to decide how much traveling you plan on doing and think about comfort compared to 2 or 3 MPG difference if even that much. Another thought is, when you get all set up with a camper in the back of your truck and are ready for a good sandwich after traveling the day and getting set up, and you discover that the loaf of bread is on the cabinet at home. Guess what, if you want bread, you might have to break camp to go get some bread. Been there done that, will take the trailer any time. Next decision, check the specifications on your truck, is it capable of the 4900 lb trailer ? Back to your original question, I doubt you will experience much difference in fuel mileage between the two. Good Luck in your decision and your travels.

Thanks, point well made. 
Jim

We had a small truck camper for awhile and then a 20' TT.  Fuel mileage was pretty much the same for both.  The wind resistance and weight on the truck with the in bed unit used as much fuel as towing the TT.  I was towing with a diesel so my experience may be a little different from yours.  We now have a fifth wheel and still get close to the same mileage because it tows so easily.  I will say that winds will affect any of these units dramatically as far as fuel consumption and handling.

Thank for the onput
Jim

ooops Input

Like others have indicated, and this is just my opinoin no sientific study to back it up, I feel that its the wind drag more so than the weight that effects fuel mileage. Like Dewwood said bigger trailer, same frontal surface area (wind drag) nearly the same fuel mileage. So your fuel mileage should be close to the same with either. Probably, if things are kept within reason you will get in the 10 mpg range maybe a little more maybe a little less (count on less to be prudent). Lots of factors come into play, one of the biggest being speed. Typically a little slower means a little better fuel use. Keep weights in mind but buy what fits your situation the best. People have done fine with both. Flyone

Another thing to keep in mind, is livability. A travel trailer/towable camper will have a better interior layout for comfort. We had a 12ft camper on a truck when we first started RVing and one of us would have to sit while the other moved around. The travel trailer was much more comfortable. Especially when you get stuck inside for several days due to bad weather. Cost wise you'll get a lot more RV for your dollar by going with a travel trailer compared to a camper.