BBowers,
It sounds like you are already treating your fuel, enough. The Power Service should be plenty & the MMO is just an "insurance policy" to make sure you have enough additive.
The one thing that I noticed when starting to use an additive was that it took quite awhile (months) before it started to work. I took a trip, with my trailer, from Phoenix to Palmdale, CA. After the trip, I noticed the injector knock had stopped. However, it did come back, a little later. Then went away, again. Since then, I've started using MMO, on top of the Primrose 405.
The 15,000 mile gear oil changes, in my opinion, are overkill & probably designed to make extra money for the dealers when you choose to have them service the axles. However, the first gear oil change, I believe, is a good idea. Get all the wear-in junk out of the axles. DC charges an arm & a leg for their gear oil. It's relabeled Texaco gear oil. I called American Axle & spoke with them about the frequency of gear oil changes recommended by Dodge. They thought the first one was a good idea, also, but 35,000-40,000 mile intrevals, afterwards, should be fine.
If you are changing out your synthetic motor oil like you would a conventional motor oil, you are probably not getting bang for your buck. Many of the conventional oils, especially the CI-4+ rated, are very good & are approaching the level of synthetic quality. There are a number of Dodge/Cummins with a million+ miles on the engines, without a rebuild. And that's using earlier conventional oils that weren't as good as today's oils. Chevron Delo 400 is available through Costco for less than $43. 00/case. That's $21. 50/ oil change, for the oil, itself.
I had a used oil analysis done at my last change with 8303 on the Chev. Delo 400 & it still looked good to go. I know some people stretch it out even further.
I'm sorry to hear of your unhappiness with your truck. If you have the ability & time to do some of these services yourself, you will save quite a bit of money. At least the dealership won't be eating your wallet, alive.
I hope your luck with your truck changes in the future.
Joe F. (Buffalo)