Heat a large pot over medium-high heat and stir in the ground turkey and onion. Cook and stir until the turkey is crumbly, evenly browned, and no longer pink. Drain and discard any excess grease. Add the mixed vegetables, tomatoes with green chiles, ranch beans, chicken broth, taco seasoning, and ranch dressing mix. Bring to a boil, then reduce heat and simmer on low for 20 minutes.