Is this an issue I should dig into?

It only drops to 700 rpm when temp gets warmed up (and in real cold weather that can take several minutes). When the engine is in closed loop (warming up) it runs relatively rich somewhere between 1000-1200 rpm. Several minutes of that a couple times a day can easily rob you of mpg to where you'll notice it.

And 3.4 engines with some real mileage on them are a lot less than your 25 mpg, especially once you put slightly bigger (or even just slightly heavier AT) tires, a lift, etc. on them. I get 14-16 mpg (usually closer to 14) under normal circumstances and see 12-13 mpg in the dead of winter with the added idling time.

 

^^^ I don't warm it up several times a day. Only first time out and only if below 30F and sometimes not even then. If warm up killed my mileage by 50% I wouldn't do it.
I can get 25 with the 2.7 in summer (as I said originally) but 22-ish is max in the 3.4 in summer on back roads. 19-20 on 3.4 is normal for me. No lift, stock all the way around, not oversized tires. 170kmi on both trucks. cheers~

 

OP said he warms it up at least twice a day and has a lift with winter tires. Never implied what you do. Cheers