GFCI breaker problems in trailer
We can't guess.  It's something to do with your neutral hygiene.  You have circuit neutrals that are interacting with other neutrals.  We would need photos of all the wiring in all the boxes associated with any one failing circuit, which in your case is pretty easy since your circuits are so somple: the wiring at any failing outlet, and the panel itself.  I bet we could get to the bottom of it right quick. 

Also, make a mental note.  Next time you're building a panel in a vehicle, "plug-on neutral" is not a feature you want.  "Plug-on anything" is not a feature you want.  Next time go to the Siemens, Square D or Eaton dealer and get their panel series that use bolt-down breakers.  Normally those are used in factories where they don't want line workers tampering with the panel (the bolts are energized at all times). That's wrong for a homeowner, but the generator makes that not a problem.  You want it because it's a vehicle and stuff vibrates out.  You'll have less trouble with that.