If someone told you that you had a def tank, I don't think I would listen to any more of their advice. DEF didn't come out (in the US) until 2010 and only about half of the diesel coaches had it. 2011 was the first year that all were equipped with it.
Personally, I like DEF. I can run my finger around the inside of my exhaust pipe and will not get ANY soot on my finger. It burns clean without a trace of black smoke. I typically use about 1 gal of DEF per 100 gals of diesel.
The best part is that it reduces the air pollution.
Ron, Sandie and Lilly
2013 Tiffin Phaeton 42LH 400 ISL | 2011 GMC Terrain SLT-2
Roadmaster All Terrain | US Gear Unified Brake System | Pressure Pro