taking an Amish Surge heater out of the cabinet ?

My Amish Surge heater was purchased in 2007 and has worked fine for us in our RV. The fan motor needs serviced or replaced because of noise.
I have not been able to remove the heater from the cabinet. There are four angle brackets holding the unit in the cabinet. I have removed the screws holding the bracket to the cabinet but the brackets keep the unit from sliding out of the front of the cabinet and I can't get to the screws holding the bracket to the heater.
It is such a tight fit in the cabinet that it can't be raised up enough to clear the cabinet in the rear so removing the unit out of the back won't work.
Does anyone have a solution to this problem?

Thanks