2007 freightliner columbia leaking coolant underneath sleeper

Hello, hope ya'll are having a good weekend so far. I have a small situation that i know you guys can help on.

Its a freightliner Columbia, 2007 model, it has a coolant leak, location of leak is underneath the sleeper. I had the same problem last year while working as a company driver down south, and it turned out to be the sleeper heater core that time on the 2005 Columbia i was operating..so i went with that idea and bought a heater core and acouple o-rings from the dealership. The tech @ freightliner quoted 8 hours labor @ $97/h to install a 50$ part. I just don't have that kind of milk money to throw around so i said thanks but no thanks on the install..it took my mechanic in dallas 2 hours to install but i am nowhere near that area right now..The T/A says they wont do it..something about not working inside the cab of the truck..my question is 1. is this something a somewhat mechically inclined person can do with a few simple tools 2. is there a workshop manual available if so where can one find it online...i tried the freightliner website but nothing on installing a heater core...any advice is appreciated even if you tell me i am way over my head on this one...thanks in advance...

 

temporarily stopped the leak...no shut off valves on this truck so I took off the hose that feeds coolant into the sleeper and fed it directly back into the engine..now there is no coolant going in or coming from the sleeper..