2003 Maxima (original owner), 285,000 miles, regular fuel for its entire life. New NGK spark plugs about 3,000 miles ago.
Suddenly while driving at least 50+MPH the engine started pinging really bad (it's always had just a little spark-knock with slight gas pedal pressure) but is severe spark-knock. Engine lost allot of power, barely being able to climb a big, long hill (I live in CO at 8700 FT elevation so used to a typical 25% loss of power due to altitude).
Engine starts great, smooth idle, runs somewhat OK (no severe pinging) (but noticeable lack of power) until pulling hills on top gear. I can hear the engine intake "groan". SEL was blinking for a while, then went off. SEL blinked during each outing but when off. Autozone couldn't read code because SEL was not on at the time. Later in the day, the SEL came on and stayed on - Code is PO300 "multiple misfire" but I do not feel misfires. I replaced air filter, added Techron total fuel system cleaner to 1/2 tank of gas, no change. Sprayed starting fluid around vacuum lines (with engine running), no change, so likely not a vacuum leak. Vacuum actuated butterfly valve on passenger side of intake plenum, has a very slight squeak and bind right at 90% full open
I do not want to put a bunch of $ into the car since it's not worth it. I hope to fix this myself. Hoping it's something easy like ping sensor or crankshaft timing sensor.

I doubt any of the sensors would cause that except maybe the MAF. Since it happened suddenly, I'd start by cleaning the MAF. All it takes is a piece of butterfly wing to bring any Nissan to a grinding halt or cause all sorts of misbehavior. If that doesn't help, look for a come-and-go vacuum leak like a broken intake boot that's opening up when the engine rocks in the mounts. If you don't find any, do a voltage-drop test on your engine grounds and see if you have a degenerating ground cable. If none of that helps, then you'll need to get a streaming scanner and see what's going on with the mixture and timing parameters.

Hello!
New NGK Platinum spark plugs about 3,000 miles ago and new plenum gasket. Sprayed starting fluid all around vacuum lines and intake boot - no change in rpm. Put new engine air filter in
I have run Cataclean through it. Now in the tank (premium octane) and a bottle of Dura Lube Severe Catalytic and exhaust treatment. I have seen no difference in performance yet. Engine has severe loss of power. Starts idles and revs smoothly in (AT)-neutral. Low power until about 2500 RPM. Above 2500 RPM it just bogs, some slight misfire felt. (did a code check with blinks on SEL = 300 - multiple engine misfire) Runs fairly OK downhill in top gear. As soon as I come to a hill and downshift happens (causing RPM above 2500 - severe bog and won't go over 40 MPH with a lot of what sounds like "ping-rattle".
Local (independent) shop says clogged cat(s) typically don't cause multiple misfire code.
Any more thoughts?

Oh yeah, cleaned MAF sensor. It was very clean but cleaned it anyway with MAF sensor cleaner

Look into knock sensor . Try to measure it's spec in ohms , clean the connector well , see the difference plugged-in vs not plugged -in .Sounds retarde at the right (wrong ) times .
When my 93 J30 had the mystery bogg @ 2700-3300rpms , I sort of "shotgunned" it. And I bought a REAL Infiniti dealer knock sensor and mounted it on top on motor's intake , centered front to back and slightly drvr's side by 3" off ctr. (Used one of the escuctheon bosses) , I tightened with a FULL 1 1/8' thread reeaal tight . The Factory put the one sensor DOWN UNDER manifold !!! So I one-upped the Factory.
It worked just as good as ' Factory ' . And that may have been the secret to the enduring end to that bogg.

Replaced knock sensor with genuine Nissan part. No Change
Just replaced fuel pump and fuel filter. No Change
Next is to check all ground straps and clean or replace. I have conductive grease for ground connections. Just since it started running crappy, I noticed that sometimes when I go to start it I just get a "click" and starter doesn't turn. After several attempts to start it finally does. This now makes me think it could be a bad ground strap.