A 30A system should be sufficient and somewhat more straightforward and less expensive than a 50A system. You can typically run the AC and microwave simultaneously with a 30A system (provided your power source is capable of supplying 30A--so not from a 2000W generator). You can't really run another additional heavy load simultaneously, though.
Think carefully about what you want to run from the inverter. If it's only for a laptop, television, or similar modest power user, you can probably simplify things considerably by not tying it into the whole 120V distribution system. (It is practical to leave something like a TV or a laptop always on the inverter and just get a large enough converter/charger to power it when on AC power. It's not as efficient as switching to shore AC power, but avoids the need for any sort of transfer switching.)
If you think you need to run the microwave from the inverter, you'll need a quite large battery bank and heavy-duty wiring between it and the inverter. That's a lot of space and weight for batteries. Personally, I don't think it's worth it, but not everyone shares that opinion and that's fine.
Switching between generator and shore power (the transfer switch in your none to great diagram) can also be done manually, most simply by wiring the generator to a socket you plug the shore power cord into when using generator power. This is simple, inexpensive, reliable, and pretty much foolproof, but not as convenient as an automatic transfer switch.
Bear in mind that the AC wiring needs to have completely separate ground and neutral busses; they are not bonded together at the 120V panel as is done in a house's main panel. This is because the trailer panel is basically a subpanel of the house or campground when connected to shore power. An RV generator will have neutral and ground bonded together (at the generator), and the transfer switch must switch the ground line in addition to the neutral and hot(s). Portable generators for contractor or household use don't have ground bonded to neutral, at least by default.
I would not recommend using all the same roll of wire for the 12V system. Having different colors for different circuits will make it a lot easier to keep track of what is what and to troubleshoot problems down the road. Most of the 12V branch circuits could be made with the same size and general type of wire (sized to be adequate for the current needed, and fused appropriately), but the high current DC circuits between the converter, DC panel, battery, and inverter need much heavier wire than you'd typically use for the branch circuits.
Typically an RV will have a separate 12V system from the general vehicle chassis system, and a separate battery or battery bank. There's generally an isolator that can tie the two 12V systems together when the engine is running to charge the house battery from the vehicle's alternator, and often also with a switch to start the engine should the chassis battery be drained. The isolator may be a heavy-duty relay (aka continuous duty solenoid), or a diode gizmo. Running the lights etc. from the vehicle starting battery is not a great idea because it's to easy to strand oneself and also because a starting battery is designed to supply high current but not to be discharged frequently or deeply.
