I assume you mean Barney Benson - Barney Miller was a 1970s/80s sitcom starring Hal Linden.
Each episode that you named includes a late-70s brown Ford Econoline van, but there are differences. The one in 'Big Brothers, Duke' appears to be a lighter brown and has tan or gold stripes along the sides. Although '10 Million Dollar Sheriff' and 'Trouble at Cooter's' are consecutive episodes, I spotted difference there too. The van in '10 Million Dollar Sheriff' has a roof rack, bars on the rear windows, silver trim over the wheels and along the sides, and the interior of the rear is painted in a dark color. Note: When Rosco pulls it over and it crashes off-road, the van changes to one with round headlights when they are rectangular for the rest of the episode. It would be possible to remove the additions and repaint the interior to change the appearance for 'Trouble at Cooter's', but my guess is that they were all just similar vans.