Lots of good advice here. It is also possible that your AC unit has gone bad. Our OEM Dometic 15K unit went out in 2 1/2 years. Dometic gave us a free replacement if I paid for replacement labor. That one went out in a few months of fall and next spring use. On line rumor was that Dometic had problems with China made evaporators that developed pin hole leaks--pretty suspicious that I had 2 of them. Third try was an old scratch and dent from a warehouse clearance sale that was made in Elkhart--holding up fine after 18 months.
If OP is not getting at least 15 degrees drop from intake to output, it's a losing cause. You've got AC troubles. I never have gotten quite 20 degrees drop (probably a humidity thing), but it does keep up.
Len R