Need advice from someone EXPERIENCED with new freightliner glider kits

My husband just sold his first wife (2000 Peterbilt 379 EXHD) for a 2013 Freightliner Coronado Glider Kit. The purpose was to get better fuel mileage. We were told this truck could get 5 - 7 mpg depending on what we are doing with it. He hauls feed locally so we know we are NOT going to get 7 or even close but we were hoping for 5 - 5.5. So far it got 4.5 mpg bob-tailing 3 hours home from the dealer and it has been getting closer to 3.5 mpg while in use hauling feed last week. He can't get it to go over 65mph. It acts powerless, like it's not getting fuel. Freightliner in Des Moines refused to touch it at first and told us to take it back to the dealer in Waterloo. The dealer called Freightliner in Des Moines and got that straightened out so we dropped it off Saturday. They called him today and told him there is nothing wrong with it. We are wondering what we have gotten ourselves into. Any advice is welcome. Keep in mind this is not our first rodeo. We have been in business 7 years and my husband has driven a truck and worked on trucks for 15 years.

 

Yes it is a 12.7 Detroit and it supposed to be 500hp but it is only reading 400hp at the shop which they say is normal. They are telling us "sometimes you just have to drive it until something breaks to see what is wrong with it." My husband told him we don't want the $700 towing fee that will come with that option. They are saying the low mpg reading is because he lets it idle too much. The reading was 4.0 from home to the shop which is 90miles away. He bob-tailed up there and didn't sit and idle at all and the average mpg was 4.0. He says it has a "miss" in it but of course no one from the shop has gotten in the truck and driven it to see what he is talking about. Conveniently the dealer we bought it from in Waterloo isn't answering his phone today. We are still waiting and praying for positive results because we are a small company and we cannot afford this truck if it gets worse mileage than the old one.

 

That would be great! Thank you! My husband is just sick right now.

 

We purchased it as a powered glider from Harrison Truck Centers in Waterloo. I guess I don't know enough about if they are an actual Freightliner dealer but the way I understand it, the motor was put in at their shop in Waterloo. We were told by the salesman that the 3 year warranty is to be honored at ALL Freightliner dealers. He said "this truck is classified as a brand new truck."