OK! I completed the Oasis flush today. Took several hours, mostly in getting pieces of hose to fit my pump. I should have spare RV hose around, but I don't. Add it to the list.
I had called ITR (they answered my call!

) and they said that I could just use a pump to pull the fluid out and then push new fluid back in. That did work well, but I did have to disconnect both heater lines (one at a time) and push some low pressure air through to get out several more gallons of fluid. The fluid was a nasty color of brown... I also found that my bucket that I pumping the fluid into had a small hole {sigh}so hopefully it wont kill the grass (weeds).
Once that was done, I then could reconnect everything, reverse the pump hoses and push new fluid back in. That part went quick.
As you can see on the NE-S, everything is right at the door, so I do not have to crawl into the basement to get there.
Anyway, I don't see any leaks, which was one of my concerns and I was able to run it with all three pumps running and it got everything hot. It took 18 gallons, although I probably went about gallon too high as the overflow tank dropped a bunch on the ground. Oh well. The NE-S takes about 14 gallons, so the other three were the heating lines. I have two gallons left, which I will keep as spare.
Glad that's done. Not sure I would want to do it again, but I did learn a bunch in the process.