My truck sits lower in the back just a tad bit, even when the trailer isn't on it. But a true "all seasons" trailer is much heavier than a regular trailer (and many claim to be an all seasons)of the same size due to the thermal pane windows, added insulation in the roof, floors and walls, etc. The GVWR on my little trailer is 11,500 lbs. so it does squat a couple of inches more than stock position, when loaded.
Mileage without the trailer is average of 15-16 mpg with the trailer it's around 12-13 mpg. I've been accused of being an agressive driver but I'm pleading "not guilty" your honor.;)
I did have a situation up in Redding where my light turned green and I started to pull out when I realized that a tractor/trailer coming down off the hill wasn't going to be able to stop at the light. I nailed it and even with my trailer on the back, I spun those 20's for probably ten feet. I was totally surprised at that....it just speaks to the torque the diesels have with those twin turbos.