What does the judgment say? It should state that you are entitled to interest at the "Federal Rate" purusant to 28 U.S.C. 1961 and/or it may have a specific number.
Your collection attny is correct. If the Chapter 13 Judge ordered a higher rate during the course of the 13, once the case was dismissed, that Order no longer had any effect. See 11 USC 349(b)(3) and In re Nash 765 F.2d 1410(9th Cir. 1985).
Des.

