It varied by brand and model as well as by chassis. All new on-highway engines manufactured after 1/1/2010 were required to meet new EPA emission standards and for almost every diesel that meant a DEF system. However, chassis could be built in 2010 using engines from 2009 and so some early 2010 chassis do not require DEF. Further, motorhome manufacturers buy chassis ahead on time and have them on hand, so more than a few 2010 model motorhomes do not require DEF.
Navistar tried to build diesels without DEF but failed to meet the emissions standards and I thnk all those engines were recalled. Few appeared in motorhomes anyway. A couple of the smaller Commins diesels met the EPA spec without it as well. For example, the Cummins ISB used in Dodge Ram trucks did not utilized DEF until the 2012 model year.
You may be thinking of DPF, which was used in 2007-2009 diesels. That is a Diesel Particulate Filter and is a different animal than DEF, which is the common name for Selective Catalytic Reduction systems, which use DEF in the process.
Our coach is a 2011, and has a DEF system. Like Marty and Diane, we've had no trouble with the system at all. It's only a minor PITA to fill every few thousand miles, but even that's gotten easier with more DEF dispensers at truck stops.
When we were looking at coaches, I researched DEF systems, because I knew nothing about them. My first instinct was to buy a 2010 non-DEF equipped coach, because in my experience, pollution control devices usually caused mileage to suffer, and other engine related inefficiencies. I learned, however, that DEF systems were so effective at treating the exhaust, that manufacturers could re-tune their engines to be more efficient, and ultimately get slightly better mileage.
I wish I could do an apples to apples comparison with an identical non-DEF equipped coach, but that's obviously not practical. In any case, we've had no problems at all with our DEF system.
Yeah, the 2010 ISB was able to meet the new EPA emissions specs in at oeast some configurations without using an SCR-DEF exhaust system. I think the 340 hp ISB didn't change over to SCR-DEF until somewhere in the 2011 engine production year. There are many 2010 model-year ISB rigs that don't require DEF, and at least some 2011's. From what I understand, the lower output ISB (305 hp or less) could meet the EPA spec but in later years the 340 & 350 hp versions did not, so eventually they all went to SCR systems with DEF. I think the ISB variant used in the Ram trucks was the last one to get DEF in all models and it was for the 2013 model year, but some Ram models changed over to DEF in 2011. However, school buses equipped with an ISB got DEF in 2010. It gets pretty confusing with the ISB! Maybe somebody has more definitive info on just when the various ISB engines changed over.