Nope, they have AC thermometers at advance for 4.99 or if cash is tight a kitchen thermometer will work just fine. When you figure it out let me know...what you described is just like what my system is doing, it also does really well when it's below about 85 or 90 but above that it just makes life bearable not really cold even on max AC, and this is with new refrigerant, evap core and condensor core cleaned out, new orifice tube, new liquid line, new orings and new oil in the compressor. I'm told the old R12 cores are not efficient enough to really get as cold as they can with R134a. Not sure if this is true or not. I still should have the heater core shutoff mod. I'm pretty sure my system is fully charged. I'm told a variable orifice tube will get you colder too, but nobody explained how. I do know my high side pressures are pretty high on the manifold gauges or at least they were. Sorry for the highjack, but what applies to your truck probably applies to mine and vice versa.
bbbrrrrrrrrr 
