Air conditioning & power loss

I have a 2006 Dodge Heavy Duty Ram 2500 diesel 4x4 truck.  I recently had my truck serviced for the second time at the same dealership.  The first time I had to take it in because of a rear end problem.  When I picked it up I immediately noticed a huge power loss.  I brought this to the dealership's attention but they claim I must have received a bad tank of fuel.  The second time I took it in the air conditioning was only blowing cold on one side.  They said it needed recharging and that my power loss was due to a dirty air filter and I "possibly" needed a "fuel filter" and I might "think" about changing the oil if I hadn't done that recently.  The air filter was not that dirty.  I went ahead and replaced both even though I just replaced them at my last oil change.  My air conditioning is a little better but my power sucks!  I can floor the thing and it won't burn the tires off at all.  Before I could burn them off in fourth gear in the rain.  I actually had to be careful where is now there is no worry of that even happening.  Not that that is how I normally drive my truck but that is an example of the power loss I am experiencing.  What do you think happened and how can I repair this?  The dealership will not take care of it.  Also, I have lost approx. 4.5 mpg