While a 3rd A/C maybe your only solution. Insulation is what you are lacking.
My 39' unit does well with 2 A/C's. Granted my unit in not quite as big.
But I have an insulated ceiling.
Don't get me wrong my unit is nothing spectacular. It was slapped together like most RV's. But is does have some insulation and does pretty well in the heat and cold.
We have been good into the upper 90ºF in hot and humid Maryland and good down into the teens in the winter.
We generally heat with space heaters in the winter and use the furnace asw auxiliary back up.
In the summer 2 A/C's do the trick. I have never wished I had a 3rd.
I imagine adding insulation at this point may not be possible. But if you could find a way to add insulation it would go a long way.
Adding a 3rd A/C maybe your only solution but without adequate insulation it may still be struggle.
