MPG question

[LeadHead]

We are planning on a family vacation this summer and it will involve nightly camping at national and state parks along our route. We'll be covering approximately 5,000 miles by the time we get home. Our 2003 Yukon XL with the 5.3 engine gets about 15.5-16 mpg with a normal load of family and luggage. Of course, traveling that distance will make it difficult to have enough room for all of our gear without some sort of trailer, so I am considering buying a lightweight popup camper which weighs around 1200 lbs empty. Considering the low profile (it'll be lower than the roof line of the Yukon) and relatively light weight (maybe 1600 lbs loaded), does anyone have an educated guess as to how much MPG loss to expect? My driving habits are mild and fuel-saving as much as possible, and our budget will be a limiting factor in our decision.
Anyone out there in my situation with some experience?

 

[b4z]

From Fredericksburg, va. To charleston, sc. I got 14.8 mpg towing. 5x8 single axle uhaul with 2 heavy dressers, chairs, bed frame. My guess is your load will be a bit heavier. I drove 72 mph with a lot of slow accelerations to 75mph so as to not hold up traffic when passing trucks.
I was by myself and did drive for mileage. '04 with 3.42s.
And heavy ltz 20s.
My record by myself and not towing anything driving below 68 mph is 21.0 mpg.

 

[LeadHead]

If memory serves me correctly, this truck has 3.73's. 16.5 is about the best we can expect from it.

 

[b4z]

Yeah, I was going to ask you if you had 3.73s.
Couple of years ago we towed our utility trailer with a queen mattress set standing up to our mountain house in NC at 2300' elevation and got 13.1 mpg.
My guess is your worst case is 11 mpg. But more likely you will be in the 12 range.
The 3.73s will help with keeping the truck in overdrive more. The only suggestion I have is to drive slower than you normally would.