Wow...you got lucky. An RV tech who knew how to add a service port. Unless he screwed in a bullet valve, which will eventually leak anyway. AC cycle does not consume refrigerant. Low refrigerant means a leak and I would bet the the unit isn't worth fixing. However, if it runs for a month or two, you can keep adding refrigerant yourself.
if I could ask, what did he charge you to come, put in a service port and add refrigerant? Just curious.