Do you need a DOT card?

I do. As for whether you do or not, that depends on your situation. If you have a CDL, you need one. If you operate a commercial vehicle of 10,000 lbs. GVWR or greater - even without a CDL - you need one. For RVs, farm vehicles, and other types exempt from requiring a CDL by the federal government, that's a state level matter, and your state's DOT would be the ones to determine this.