Welcome from the Florida Panhandle
First off, welcome to the Forums. We're glad to have you with us.
Sorry to hear of your towing problems. I'm sure they are something that can be worked out.
I pull my sweet Lucy (2005 25FB) with Olivia, who is a 2005 Suburban 2500, 4x4 with the 6.0 liter gas V-8, 3.73 rear ratio, and 4 speed auto transmission. I have pulled with this combination for over 30,000 miles all over the United States. Because I like this tow vehicle so much, I recently purchased a second Suburban named Daisy. She is a 2004 2500 equiped exactly like Olivia except she is 2 wheel drive. Our towing experience with this TT/TV combination has been excellent Olivia (the 4x4) gets in the high 9's mpg when pulling at 70 mph. When I slow it down to 60 mph, I get in the mid 11's mpg. Daisy (the 2 wheel drive) got 12.8 mpg on the one short run that she has been on.
I did a little reseach on chevrolet.com and found that the 2008 Suburban 2500 comes only with the 6.0 liter gas V-8 and the 3.73 rear end ratio. Since the 2008 has the new six speed auto tansmission, you should be pulling better than me and getting the same or better gas mileage. You are pulling the same Airstream with a better edition of the same tow vehicle.
I do use a Hensley Arrow Hitch, but this should not impact pulling capabilities or or gas mileage. It sounds like your rig is not set up properly. Sometimes RV dealers know less about towing than they do about brain surgery. I am not familiar with your brand of brake controller, so I can't comment on the setting. Read your owners' manual thoroughly and it should tell you where it should be set. Use the manual lever on the brake controller to get a feel for how it is working. Do this on a back road without ant traffic. The trailer brakes should not drag at all, yet be able to stop the whole rig without using the TV brakes at all.
Another factor that may be contributing to an unpleasant towing experience is that whatever hitch you are using is not properly set up. If you would provide further info on your hitch system and post some photos of your rig all hitched up, someone here on the Forums will be able to help. You may be set up with too much or too little tongue weight or not adjusting the WD bars properly.
I am assuming that you, in fact, have a weight distributing/sway contol hitch system. Any trailer the size and weight of a 25FB MUST have a hitch system to be safe on the road. If your dealer failed to make you aware of this, give him my name and telephone number and tell him that I said he is an incompetent idiot.
You have a great travel trailer and a great tow vehicle to pull it with. The problem must be in the set up.
SuEllyn & Brian McCabe
WBCCI #3628 -- AIR #14872 -- TAC #FL-7
2015 FC 25' FB (Lucy) with HAHA
2005 Suburban 2500 Quadrasteer (Olivia) & 2011 Silverado 3500 (Fred) with Outfitter Truck Camper (Ethel)