I have broken/ interrmittent running lights
On2000 box tr.
I don't know if it's the fuses or bulbs
I have checked under dash fuses w/ no success
What's my next step

Are the running lights the only problem? Is there a certain time you notice that they don't work?

You need a volt meter to verify that there is 12VDC getting to the lights. If there is, then you know you have a ground problem. If not, then you need to check to find out where you are losing the 12 volts after the fuse.