Is a car dealers service dept responsible for repairs to my engine?

I purchased a 2002 Acura TL from a local Toyota Dealer in 2007 and at the same time purchased a maint agreement for the auto. This takes care of oil changes tire rotations etc for the life of the auto. This is the only place that services my car because I am not a mechanic therefore the purchase of maint agreement. My records are impecable and show every service and every gas fill up with miles date gallons and total price. Call me anal anyway my son was going to take the car to ohio for the summer so last Fri took the car in for oil change and received the service. On Sat morn my son and his buddy left for Ohio and made it 151 miles up the interstate when car started running hot and he immediately pulled off the interstate into a convience store/mechanics shop. The car overheated and was spewing from the radiator. After ir cooled off they opened the hood and the mechanic noticed the radiator cap was not present and asked when was car serviced last and my son informed him the day before. The mechanic stated they probally left it off or loose after the service so the mechanic drove 11 miles to auto zone to pick up radiator cap plus anti freeze and returned and filled up radiator with fluid and water and put cap back on. The car at this time had ample time to cool down and when they started car smoke and oil and water flowed out of tail pipe and engine sounded rough. The mechanic stated it was a blown head gasket for sure and the heat probally warped the heads which means rebuild or drop anothe motor in. I called Toyota dealership and informed them of what happened and was told by service mgr its not their fault and besides he says you only came in for oil change. He states they do not do inspection under the hood for fluids air filter wipers and such. I called him a liar because they try to sell me these items everytime I am there. O not this time huh how convienent. I have taken pics of radiator and the neck is not bent or twisted the hoses are fine and no tears in radiator. Do I have any chance of getting a judgement to pay approx $2750.00 for another motor. Does it just come down to my word against their word and is it worth trying litigation. This is in the state of Alabama.