There used to be a radio commercial for some product -- I forget what,
and that's supposed to be a frequent problem with clever commercials;
but I digress -- that started out by asking people, "Would you rather
play this tuba full of mayonnaise, or..."

My humble guess is that that's what your engine is doing, if you have
110,000 km (70,000 miles) on the original air filter.   I'm impressed
by the  range of the memory map in the engine computer, but am
reserving judgements on the efficacy of  the maintenance telltales.

It is more plausible that the plugs are still decent but it probably
won't hurt to change them.  Yank one or two and see what they look
like.  Think about the maintenance interval of the plug wires too.

I basically never preventively replace an 02 sensor -- just wait'll it
throws a trouble code.  (By the way, On-Board Diagnostics II scanners
are your friend -- there's even at least one now that doubles as a
driving computer of sorts, so you can see what sort of mileage you're
getting under different conditions).  The thermostat I replace during
routine replacement of radiator hoses and coolant, whenver that either
comes up on a schedule or strikes me as a good idea, whichever is in
nicer weather.

I'm rather bothered by the idea that you've gone through three trannies
in 70,000 miles.  Que limon!  Or are you using it to pull/haul really
heavy things all the time?  That too reduces mileage a lot.

Anyway, start with a new air filter. They're pretty cheap, and engines
suck a lot of air, and the air is dirtier than most of us realize.
Even if you still have other problems to look for, that alone will
help.

Best of luck,
--Joe