My comment sure got quoted. It must have been popular.
I will say that air can cause havoc in hydraulic systems. Ever had air get into your cars hydraulic brake system?
RV's get hot in the Texas sun, but not hot enough to make the transmission fluid act up in my rv slide system. None of my friend's rigs in Texas either.
Slide cylinders arms can get dirty and damage seals. Then they can leak. Usually air will often get in before the larger molecule oil can get out.
Again, just my 2 cents but I am not trying to be the definitive answer. Just trying to help a fellow rv'er with what I would look for. I think I am done now.