Crazy MPG that continually varies

One minute im getting 50MPG, next time 25MPG, all based on the dash info. I regularly drive from LA > Sacramento and usually average ~45-46MPG while doing about 80mph most of the way. The past two trips, I couldn't even make it to Sacramento without filling up which wasn't a problem in the past. I am currently on a road trip and am traveling 10K+ miles and the best I am getting right now is about 36MPG. . It's pretty hard to describe exactly what's happening because I don't truely understand but I will give it my best shot. It almost seems like the engine and the transmission are not in sync. It seems pretty decent when just running on battery but once the ICE comes on it's a whole different story. The vehicle seems to really drag and seems to rev higher, burn more gas, and not produce as much power, but it doesn't happen every time. I can be at the end of my drive getting 25MPG, pull over, rest for a bit and start going again, and now it's up to 45mpg or so. There will be times when I'm doing about 50MPH and the ICE is on about half way in the ECO bar/range and its getting 45MPG and all of a sudden it will just drop to 20-25MPG without making any extra power or noise. Also, I some times find that even going down hills I need to accelerate to maintain my speed and even then the MPG will drop to 30ish or so. Brakes aren't dragging and there is no extra heat from them. My load is light, just me and some luggage. I'd really like to figure this out ASAP considering how many more miles I have on this trip and the money I can save with the mileage I am used to. Note: Speed on this trip is ~72MPG. This has been on and off for awhile and since thdn I have put in brand new plugs and swapped tranny fluid and coolant with same results. Thanks.

 

Tires normal.

 

I don't remember any of this behaviour before. This is between about 25-30 tanks and getting 40mpg max. Averages before were 48-51mpg.

 

I have the obd2 Bluetooth reader that is pretty much always hooked up and no codes.

 

Tried without it and didn't change. I mainly leave it in to log the vehicle while driving since most of my drives are 9+ hours.

 

It is not - I still get about the same amount of drive time on a charge as usual. About two weeks ago I ran the light bulb test on each cell of my HV Battery just to be sure it was good before I left on this current trip and all checked out normal. BTW - Thanks for the Replies.

 

UPDATE - Made it down to Florida and got pretty terrible mileage from Boston down and went about and pulled the plugs and all were covered with oil. Gonna try and run a compression test but since these damn plug holes are so small I'm not sure Im gonna get the right sized adapter to properly run the test. With that being said, what are the chances of it just being the piston rings? Pushing pretty damn high mileage and pretty much all highway use.