air cond troubles

got camper out of storage today and turned air on and it worked good for a couple hours then I noticed that it would not cycle off when the desired temp was reached it would just keep running

funny thing is i have my thermostat set on 75 and when i turn it down lower then the air cond. turns itself off completely

also when i first turn it on the compressor starts immediately when usually the fan starts and the the compressor starts about 15 seconds later

is my thermostat bad???? ps its 20 years old and never had any problems with unit and it still blows 45 degree air out