I'm a relative newbie, but I like the idea of sticking with an antifreeze flush rather than air, as the system was built to transport fluids, and at least you know and can see that the antifreeze has gotten everywhere it should. If you're familiar with the siphon technique, stick with that..and if you can't locate what is supposed to be there, just put your own in a convenient place. Mine was plumbed between the storage tank and the pump, and they you simply empty the freshwater tank. I also don't like the idea of putting antifreeze in the main tank, in case a funky taste sticks around longer than I care to deal with (I know there will be a little in the potable pex lines, but the surface area is much smaller and maybe they are more resistant to anything objectionable adhering to them (purely a guess).
FYI, the only mistake I made last fall, was the very last step when depressing the valve for the freshwater hose input...my directions from an RV place were clear, but I neglected to turn the pump off first and use residual pressure to let antifreeze exit. With pump on, the pressure was so great, that it almost blew out the o-ring seal entirely (dislodged it enough so the system now won't hold pressure until I fix that little assembly).
Like the OP, I too find the 'owner's manual' next to useless, because they make them for at least 4 different models in my case, but only print one manual. That is frustrating, because you end up with generic information only, and much that doesn't apply. For the price of these units, that should be the standard practice to give a decent manual...and it looks to be a common issue.
Good luck with your search for your siphon! Maybe should call up the salesman or his techie.
Bob and Manola
07' Four Winds Siesta 27', 2 slides