I've ran it in my 12 2500 CTD about 3 times. Never actually filled the tank completely with it though. Once I only put about 10 gals in and the last two I probably put about 20 gals in. Truck holds 36 or so gals IIRC. Have not had one problem from doing that.
One time when I was at the dealer and talking with the service tech I asked him the same question. His answer was that he thought it was more for the DPF getting clogged than anything else and to make sure you keep an eye on your fuel filter. He also suggested that the fuel filter would probably need to be change more often since the 20% will clean all the built up crud and the filter will get filled with it.
I run a dual Cat filter setup as well as the OEM, so I have 3 filters. I have about 5-6000 miles on the truck since changing filters. I have about 2000 miles on it since running 20 gals of 20% thru it. Still runs the same. I have no problems running 20% if needed. But I won't run 20% on a regular basis.
This from the Cummins site.
Is B20 approved for the Dodge Ram Turbo Diesel?
Cummins approves the use of up to B20 in the Dodge Ram trucks for municipal, government, and commercial fleets only. This applies to selected model year vehicles. Please consult Chrysler for specific requirements and approved vehicle models.
Not sure what the difference is between a comm fleet, government truck or a standard Ram bought off the dealer lot.