Orders for new trucks, buses, and RVs in classes 5-7 reached 222,200 units in 2014, an increase of 12 percent from the prior year, according to ACT Research.

The month of December was also a particularly strong one for orders in these classes, as orders rose 44 percent compared with the same month in 2013. The month was the second strongest of the year, said Kenny Vieth, president and senior analyst for the firm.

Some of the December strength could be attributable to federal legislation that raised the Section 179 bonus depreciation limit to $500,000 from $25,000, said Steve Tam, ACT's vice president for the commercial vehicle sector.

"Besides that, it goes back to the fundamentals," Tam wrote in an e-mail. "The fleet is old due to delayed replacement; the economy is strong; businesses and municipalities are feeling good about their prospects, and they continue to act based on those factors."

Tam said he expects "measured growth" in the new medium-duty market in 2015.