Side air conditioner problems

I have an older model side air conditioner. It is very inefficient. I know the newer units have a roof mount air conditioner that are more efficient. However, my unit is too old to support installing one. Anyway, does anyone know how to make it more efficient? I can buy a new air conditioner, but I think the design of the return air hitting the bathroom and returning is messing up the thermostat. Advice please????