The predominant form of gunk stuck to the walls is cellular debris and metabolic products of fungals (various species of 'kerosene fungus') and other microorganisms that use the oil as a nutrient source. Most of the 'black' crap will be 'asphaltine' similar compounds --- cellular 'poop'.

Fill the tank, add a commercial 'tank cleaner' (Starbrite, etc.), let long term soak; then drain the tank, open the tank (via - manhole or handhole) and with scrub brush mounted to long handled pole, knock all the crap to the bottom, add absorbing type concentrated 'garage floor cleaner' and let it soak up the 'soup'.
Then mechanically remove the bound sludge with small shovel or large 'putty knife', etc.; use fresh fuel to 'sluice the walls' and then sop up with paper towels (burn the towels) and remove all visible remnants of sludge.
Take the drained/pumped fuel home and burn in your oil burner or discard at a used oil facility.
Fill tank with a few gallons of fresh oil and recirculate polish (via independent polishing filter set and pump) using the discharge of the recirculation filter to 'wash' the walls. Re-sop remainder with paper towels, etc. etc. then fill tank with fresh oil. Goal is return tank walls to 'bright'.
Run engine normally for a few weeks, as normal, then change out the racor, etc. filters that remove any residual debris - change immediately if vacuum gage shows increasing values .... you dont want this softened crap to 'extrude' through the racor, etc. filters, not completely burn in the combustion cambers to only settle out in the exhaust system an injection elbow causing the formation of 'coke' (blockage).

To protect fuel / tank, consider to install a small independent (~ 1-3 gallons per MINUTE 'turn-over' per 100 gallons) recirculation polishing filter (~10µM) that is run any time that the engine is 'on'. The recirc. filter set removes the spores and nucleation particles that eventually 'grow' and form the 'gunk'.
Use bio-stat with all new fuel. Buy your fuel from 'high turnover' fuel sources in the future, avoid buying fuel from low turnover marinas. Do not continually 'top off' fuel, just enough for your planned trips plus some extra reserve. Consider a large 'bio-blocking' and water (humidity) absorbing 'filter' for the tank vent.
Completely drain the fuel from the tank during long term 'lay-up'.

When taking on fuel from non-regular depots/marinas, FIRST pour the fuel into a small clear glass container, hold the glass between your eyeball and STRONG white light ... if you denote a 'haze' in the fuel as 'back-lighted' by the strong white light, close up and go to a source that has CLEAN and uncontaminated fuel.