There is something wrong with your plumbing. Water (or effluent) will find its own level in the entire system. To back up in the sink - which is no doubt at least 2 feet higher than the shower - would require the shower to be 2 feet deep in effluent. This would be more than inconvenient.
Both shower and sink should have traps, and the tank should be vented. If the backup you got was a small amount, it is possible that the vent is plugged (birds, wasps, bad construction) and the tank pressurized by a warming day, this could force the material in the traps up into the bowl and shower pan. To see the actual level of the black tank, hold the toilet flush open, on most RV's you will be able to see directly to the surface of the effluent.
