I'd imagine heat is key. Some form of detergent, soak time in hot water followed by agitation would do the trick one would think.
As a homeowner I used to find a job for the water drained from the main water heater in regular maintenance.
30-45 gls at 140F plus is potent all by itself. A specific cleaner followed up by a hose wand pressure attachment would make it even more of an attempt.
1990 35' Silver Streak Sterling
; 9k GVWR.
2004 DODGE Cummins 305/555; 6-manual; 9k GVWR.
Hensley Arrow. 10-cpm solo, 18-cpm towing
Sold: Silver Streak Model 3411