It sounds like your system is free of refrigerant leaks as you said it cooled fine last summer. Adding more refrigerant then needed will only shorten the life of the
compressor as well as do just the oppisite of what you think. With a 15 year old unit the best thing you could do is clean the condencer and evaporator coils.
This will help the unit to berath better and in turn use less electricity. Short of that, an upgrade to a unit with a scroll compressor would use less juice..
I had the same thought with my old Dometic but sooned learned there were no fittings to charge it with. By the way I took a peek at the new unit and it looks OK just a little soot and at first glance the coil appears clean. I was going to install it tomorrow but it rained pretty hard here today and I will have to wait to get it out of the yard because of mud.