6V92's TAB lack of RPM

Yep, I'd say there was a vacuum change.
10s never had a chance.
Snake oil is not going to fix that either.
Keep rotating 30 mics and stay in the choppy water to keep the Sediment up.
But I am afraid it's time for some tank polishing.
Start asking about for a fuel tank polish/cleaner guy.

That's going to smart in the pur$e but the best path.

It is easier if the tank is low, I would not think disposing the fuel is required unless it's real old and not trusted. Your fuel guy can help determine this.
Your vendor probably does not want to remove it all, need some to rinse the stuff from tanks bottoms and corners. This last fuel may end up dark. Some things ya can't filter out. You may want to dispose of this, Your fuel guy may have some better on site thoughts here also.
On full or large quantities, our vendor is able remove it and store it close with a constant filter/pump running while he rinses your tanks.

The cost of disposing of the old fuel is more than the cost of new fuel. That more than doubles the price of replacing the fuel.
He is on the left (West) coast. Under even more EPA regulations in removal, transporting and disposal of oil and fuel.
In my hood, with a mobile business, I pay a fortune to dispose of used oil & fuel. I used to sell it.

AND, You still have to clean the tank bottom and dispose of that.

It may not be bad fuel, just dirty and dirty tanks.

There we go. Your fuel guy will have two hoses into the tank (hopefully at two different places), one returning, another to pick up the crud. During the exercise, he will use the return to pressure sweep the bottom of the tank and start filtering the junk again. At some time he will make a report and ask to stop or sweep again and keep going. For my customers, I've always requested to keep sweeping till no more is picked up. This may near double the time but it's up to you and your check book.

I have worked with the polisher before,, If your buds, then go over a game plan and expected cost subject to surprises.
Show him the pictures you showed us. You of-course want that problem gone.

We are blessed with a fuel guy in Jax that works with us very well for our customers. I help give him the access to the tanks and he does the dirty work. I clean up after him and put the ship back together.
You may want to plan to make your fuel guys job quicker; Get everything (furniture, carpets, mother in law) out of the way. Open up the tanks sender opening and any service points he may need to access. He's on the clock to do it him self.

The Californian (671TIBs) we manage has a tank astern under the master bed and two engine room beam tanks. The beam tanks have side mounted level senders (pains to remove). IM me if you have these and need help removing them.

Again, your fuel guy may already know these things and if you want to help (save some pennies) he can guide you. Fuel tech guys usually don't want to move furniture, move bedding, or handle many things. You do not want him to transfer the odor of fuel to your stuff either.

Keep us up on how it works out.

Are you putting fresh oil on the gasket before assembling?
When spinning on, it stops and 1/4 turn more?
The gasket swells when wet from fuel, it will get tighter.

There are updated mounts for oil and secondary filters to make them spin on. Even for the old Allison clutches. DDC used to sell an adapter for the block oil filters when it was part of the oil cooler. AC, Donaldson and others can provide new filter heads that exact replace the old cartridge head. If your tight with a Detroit shop, ask them to find you some used spin on sec fuel filter heads.

There are usually a lil graphic or instructions on most filters. If the filter has instructions on it then that is what to follow. My comment was not to superseded any OEM instructions.
However, when I spin on a secondary fuel filter, it's pre charged (almost full) with fresh fuel, oiled gasket and spun on till it stops plus 1/4 turn by hand. Never had a problem. Never had to wrestle another one off.

I think the heat makes spin on oil filter seals expand more. I still hand tighten them but pull out the strap to get them loose a bit during changes.