There is one more on the other side too. You just can't access it.
I've never touched the drain plugs myself. I just drain the rad and drain the resovoir then start a process that will flush out all the old stuff. IMO, if you drain the block and then screw in the plug and then fill the rad, you have the posibility of creating an air pocket in the block. And, you still have old stuff in the pass side of the block, so you need to flush it all out anyway......
I'm curious as to what water temps you are reading with your sensor in that location? My guess is it's probably pretty low..... I mean, the coolant has not had any chance of running past all 8 cylinders or either head for that matter..... That's where all the heat is generated. I think you are only reading the 'coldest' temp the coolant gets.
I've got mine in the heater hose that is leaving the head on the pass side and enters the heater core. This coolant has gone through the entire engine and is just about ready to go into the core and then to the T'stat and finally to the rad. So, it'll read the 'hottest' the coolant is. My temps are a steady 190* with it going to 195* on occasion when it's hot and I'm working the engine (towing a #4500 travel trailer).
My process takes a couple hours and lots of water, but it gets the job done and I like doing it. I do it every 18 months. That reminds me, it's only six months to go till the next flush!!