Normal Lifespan of A/C Units?

My rig's in the shop right now for A/C problems. The air feels cold blowing out the vent but the thing isn't cooling. We live in Arkansas with typically hot humid summers and the first year we had it the unit worked great. 2nd Summer it has been more mild than usual and we can't seem to keep the rig cooled. I took it to the shop before my trip to Destin and they said that a divider plate was blocking part of the duct and they taped it in place and it ran fine. Down in Destin it was 85 degrees most days and even running all night the unit wouldn't cool us down to 70 degrees inside. I've always been told a properly sized unit will maintain a maximum of 22 degree difference between the outside and inside temp so at 90 degrees I should be able to maintain 68 inside.