rear right turn signal won't work 2001 diplomat LE

Since both the trailer and the motorhome lights aren't working, it's probably something in common with both, which would rule out the bulb and likely the ground connection. Either the wire for that light is broken somewhere, or a fuse is blown. (I'm assuming the front turn signal works fine, which means the turn signal switch and flasher are OK. I'm also assuming the lights don't work properly when braking either.)

some units have a seperate fuse for each side.

Bad bulb bad ground broken wire. Start at the light and work back till you find the issue