I never got around to look at how the hydraulics are plumbed back to the pump/tank on these units. But the pump will supply a manifold. From the manifold will divert the require pressure/flow (which is preset by the pump)through solenoids. During any function you will have a return of flow back to that manifold and from that manifold back to the tank. That return line is common to all functions between the manifold and pump.
Now the above statement is assuming the Manifold and pump are separate units. If that are built as one unit then my below statement is kind of worthless but maybe not. There could be a designated port on the unit to do what I mention below.
So if I was going to replace the fluid in the system. I would remove the return line at the tank and run it to a bucket. Drain the tank and fill it with the new fluid. Then function every hydraulic part on the RV to full extend and then fully retracted three or four times. You must go full travel both directions as that will ensure you are pushing out all fluid on the back side of the piston in the cylinders. Though from the factory the slide out cylinders may be limited on full travel, if so then all you can do is what you are limited to. But you have to monitor the volume in the tank as you will not be returning any fluid to the tank. As it will all be going into the bucket.
If the entire system held 4 gallons(which I have no idea of what the volume is), that's including the volume in all of the lines, cylinders, tank, and manifold. Mi would not be satisfied until I had a bucket or two totaling twice that.
Doing it this way will also ensure that you do not introduce any air into the system assuming you monitor the volume in the tank. You also reduce the likely hood of getting any contaminates in there also. Getting air in the system would or could be a nightmare.
The function of a hydraulic cylinder has two sides, Piston end and Rod End. Side of the piston that is attached to the Rod is called the rod end. Kind of self explaining. Fully retracted there is less oil volume in the cylinder then when it's fully extended. That's due to the rod consuming part of the internal volume of the cylinder when retracted. Also, every cylinder exerts a greater force extending then when retracting. Again that is due to the rod consuming surface area of the piston on where they attach. Loss of surface area means less surface area for the hydraulic pressure to react against. So if you ever have a slide that will push out but sticks on the way in that could be a partial reason. The cylinder doesn't have as much power retracting.
I have over 12yrs with practical experience with hydraulic. One thing that I have learned about oil in all regards is each brand whether dyno or man made can have dramatic effect of the efficiency of the system. Obviously the cold the climates will cause the pump to consume more energy as the oil becomes thicker. Where I live the current ATF that is in my toy hauler hydraulic system is just fine. But if I was going to switch for colder climate it would be Royal Purple without thought. Some years ago we switched all of our deck equipment on the vessel to Royal Purple Hydraulic Oil. Instantly in every piece of equipment we noticed two things. They where quieter and there operating temps dropped. Over time we also found that we had less failures from the hydraulics (this was noticeable as all repairs are logged and it was easy to see the trend over a years time). I am a firm believer in the Purple oil.
By the way, which ATF do these systems use? If its the ATF-4 I believe it only comes as Group III. Meaning its synthetic no matter what brand you buy.
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