When turning on the headlight switch on my 1993 Toyota RV, the clearance lights turn off by themselves
and the taillights dim -sometimes almost instantly..sometimes blowing the 10A taillight fuse. If I turn the the switch off and on
again everything works then just turns off again at random. The headights work normally, brake lights,
turn signals all not affected at all . Each time this happens, all the clearance lights are completely off, but the taillights are very dim when it happens. Could it be a bad relay or a ground? If so where are they located? It's a
1993 Toyota 3.0 V6 truck based RV.

 

A bad ground won't blow a fuse. You need a short in the system to do that.
A bad ground can cause dimming.
I'd look at the sockets at the tail light assemblies. If you have any tow connections wired in you might want to remove.
That unit was built on a cab and chassis unit that Toyota sold. Came with no bed. It did have Toyota truck tail lights installed on brackets. If the motorhome company had to alter that harness in any way (lengthen?) you might check their connections.

 

Turns out it was an intermittent short on one of the rear marker lights...problem solved!