... my real wondering is would something that's been laying horizontal for a number of years now, have anything in it that could be salvaged for this use?
Yes, there's likely good heart wood still there. The sap wood is likely a pulpy mess, but you usually mill that off and throw it away, anyway.
I've cut into oaks left in similar state for maybe up to 10 years, and once you get thru the 3" of punky sapwood, the heartwood is almost always fine.
Couple short comings.... have a Stihl 044 with 20" bar so that's an initial issue. No ripping chains (or whatever is used to rip a tree) but those are surmountable. Would an 044 (70.7 cc and bone stock, used a bit every year for last 20) would an 044 have the balls to do something like that?
I'm familiar with the 044. It's a bit small for a milling saw, mine was a 125cc 084, and I think most guys seriously into milling use the 088 (135cc) or larger. But with a little time and care, you can make anything work. Your main enemy will be getting the job done in reasonable time, to avoid wearing yourself out while babying the saw to not overheat it.
That said, why would you want to? It's an awful difficult way to save just a few dollars. Milled oak isn't expensive enough to make killing yourself or your nice old 044 worth all that.
Stupid question perhaps, but where would you find white oak 2x's? It doesn't seem to grow this far north, so a local sawmill probably isn't an option and I didn't see that the box stores carry it either. My trailer's gonna need redecking in the next few years.
There's a local sawmill I've used for years, Haecock Lumber. I just call them up, tell them the lengths and widths I need. In this case, it was something like eleven 2x8x16's, or "21 board-feet of eight-quarter" to the miller. If you let them do random widths, and just tell them your min length and total width sum required, it's even cheaper.
Somewhere early in this thread I posted the cost, I'm sure you can find it. Memory tells me something like $400 - $700... I don't even remember anymore. Definitely not much more than a clean replacement for
@Richard's Stihl 044, if he burns it up! And it sounds like my trailer might use 2x or 3x as much decking as his, so he might get it for $200 or less.
I'm sure there's a sawmill somewhere near you that also mills local oak. Down-side is it's usually sold green. I just put it up on 4x4 PT blocks in my driveway, tented it in clear plastic for one summer, as a home-made solar kiln. Does a nice job of drying it, in just a few weeks/months. I'll hose it down with Bora Care pesticide at some point in the process, and let that soak into the pores for another week or three, before cutting the boards to size and installing them.
Do note, this is NOT the quick or easy way to re-deck a trailer. About half way thru the process, I was kinda wishing I'd just done PT, and I've worked with more wood timber in more old houses than most guys. It yields a nice result, but it's days of work, versus just a few hours to slap down dimensional 2x8x16 PT's.