If you don't have house batteries then your only option is to connect your 12v loads to the starting battery. Generally not the ideal situation.
I would strongly recommend some kind of house battery. Even if you went with a single modestly sized 12v battery and a battery isolator so that your coach charging system will run the loads and charge the house battery when the engine is running.
Running your house loads off of your starting batteries is a great way to find yourself stranded because you fell asleep with the TV on or similar.
Trust me... I have been stuck out in the woods 30 miles from town in a rig that ran 12v loads off of the starting battery. Somebody forgot to switch the inverter off and the draw was enough to drain the battery over a 16 hour period. Time to go home and our starting battery was dead. No cell service. We hiked out to the highway, in the rain, and flagged down a passing traveler who kindly drove us back to our campsite and jump started our (borrowed) rig.
If you elect to go ahead and run your 12v loads from your starting battery I would recommend that you carry: Jumper cables, warm clothes and an umbrella.....
Edit: Also, frequent deep discharge of your starting battery will significantly shorten its lifespan. Starting batteries and deep cycle (house) are built differently. True deep cycle will tolerate frequent deep discharge (50%) far better than a starting battery.