A 15 amp circuit is probably not sufficient for the load. You need to check the circuit voltage inside the van, preferably at the outlet that the A/C is plugged into, both when the unit is running and when the compressor starts up. When the compressor kicks on a serious demand is put on the circuit and the voltage may drop low enough that the compressor will not start. The 15 amp circuit is wired with 14 AWG wire in the first place and it may be a long run to the panel. Secondly, are you connecting to the van via an extension cord? Anything less than 10 AWG is woefully insufficient. Third, you are probably connecting to a household 15 amp receptacle through a 15/30 adapter. All of these things can create voltage drop. Even if the A/C starts up under these conditions, running on an insufficient supply can be very hard on the A/C unit. It can cause the compressor to overheat, shortening its life.
