Would not blame it on the large windshield... more likely air leaks up front and the heater valve. How hot was the air from the vents and was the heater on recirculate... no outside air? Replacing the motor driven heater valve with a manual one is good, especially for increasing the AC in the summer.
Buy a IR temp gun (Harbor Freight $15) and see how hot the heater hoses are at the heater and either side of the valve. Closing the back of the MH and the heater vents so less area and more heat to the front may help.