After the patch 1.06 armored trucks still spawns after every 10 seconds - 2 minutes you destroy one. Those'll get you 2000 RP per truck. Though sometimes they might not give you RP and/or money bags.
I just do Coveted for the money (12,000$) and you get 1250(?) RP when you finish it on Easy / Normal and it takes around three minutes to do.
XBLA Duke Nukem 3D Speedrunner