I'm looking for a duramax diesel truck that can pull fifth wheel. My son, heavy equipment operator Alaska, says I should look for a used one pre DEF. He says DEF tanks and pumps freeze, and break down. I live in North Dakota, so is a concern. Also wondering pros and cons of 2500 vs 3500 (single tires on rear. Never owned a truck. Or diesel vehicle. Interested in pros and cons of short, medium and long beds too. Thanks for the input.