It might not be vented properly, or the vent could be plugged, or there could be no shower P-trap, or the floor drain is dry or it has no P-trap. I suggest you contact a plumber to diagnose this if it's beyond your skills. I'm not meaning to be insulting, it's just that this could be something simple or it could involve using sophisticated equipment such as a drain inspection camera that most homeowners don't have.

This is a common complaint of rv owners who use the rig only occasionaly. Their problem lies with the hot water standing in the pipes. It would seem that running water 5 minutes should clear it up but it doesn't. To check that,cover the drain before turning water on.