Spark Plugs Getting Ruined Very Frequently

Hello!
This question is in regard to a 2006 GMC Sierra 2500HD (8.1L gas engine)
with 197,000 miles on it. It had around 60k or 70k miles when I bought it in
2012. It has had a rough life since then - lots of hauling and towing. It has
had a long-running problem of burning oil (I would have to add some oil
every month), but the oil-burning has suddenly gotten much worse, and I am
adding oil every few days. Every mechanic I've ever taken the truck to
always says "that's just what old trucks do, they burn oil". Another problem
(which I assume is related) is that the spark plugs get ruined frequently and
have to be changed. I've had to change them at 124k miles, 130k (wires
only), 170k, 183k, and 197k. At 197k, I told the mechanic I was tired of
spending so much on spark plugs, and we decided to try a cheaper plug and
see what happens. This, in addition to a full tune-up/fluid exchanges, fixed
the symptoms I was seeing (intermittent shudder when idling, 'check engine'
light coming on when going uphill over 45mph, engine misfire code P0300).
But then I only made it 1200 miles before those same symptoms returned.
Same P0300 code, one ruined spark plug, cylinder 2. A mechanic replaced
that one plug, and I made it another 80 miles before things went south again
yesterday. This time it's 2 plugs, cylinder 2 and another cylinder (he didn't tell
me which one). So we're replacing all plugs with the better double platinum
ones. Any ideas what is going on? Am I just doomed to replace the plugs
every 15k miles from now on? I love this truck and don't want to get rid of it.
For anyone interested, there are more details below, in case they provide
any additional clues. Thanks in advance!

- oil changes are done every 5k miles
- oil cooler lines replaced in 2013
- major tune-up at 124k miles (transmission flush, coolant flush, 4x4 service
(front/rear differentials, transfer case), spark plugs, fuel induction system
service, air filter replaced)
- at 130k, replaced spark plug wires, cleaned mass air flow sensor
- at 132k, replaced fuel pump assembly
- at 169k, code P0171, cleaned mass air flow sensor
- at 170k, replaced spark plugs, plug wires (mechanic said plugs were really
bad and covered in oil)
- at 174k, replaced air filter
- at 183k, replaced spark plugs and air filter
- at 1955xx miles, replaced serpentine belt, belt tensioner, and fan clutch;
mechanic said spark plugs are misfiring/worn out (but I couldn't afford to
replace them yet); mechanic said "we took off the valve covers and looked
inside of the cylinder heads, in case the valves are gummed up. But we
didn't really find anything except the piston rings are a bit burned out." He
recommended trying Lucas oil stabilizer and Lucas fuel cleaner (which I did)
and using premium gas and higher quality synthetic oil from now on (which I
did). He said the engine is burning lean, so better fuel will help that. Told me
to get a tune-up as soon as I can.
- at 197k, 'check engine' light came on every time going uphill over 45mph;
code was P0300 (engine misfire; no other codes); idle is very rough;
replaced spark plugs/wires with cheaper copper plugs as an experiment, did
fuel exchanges (differentials, transfer case, transmission (& filter), coolant,
power steering fluid); cleaned the air filter
- 1200 miles later, replaced 1 spark plug; added "Engine Restorer 8-
Cylinder" to the oil, as recommended by the mechanic
- 80 miles later (today), replacing all spark plugs with the right kind.
Mechanic says it will take a while of driving at high speeds for the "Engine
Restorer" to work?

I travel a lot, so I rarely get to use the same mechanic. Whenever I go to a
new mechanic, they usually tell me that my last mechanic was BS-ing me
and/or didn't know what he was talking about. I'm a car dummy, so I have to
just take the word of these mechanics, and I never really know when I'm
getting the run-around.

Thanks to anyone who made it this far in my post!
(attached photo is the cheapo copper spark plug after 1200 miles)