Jack Mayer has covered a lot of what you need to know about installing converters, inverters, and solar (should you choose that), right
hereIf you're also going to install AC powered devices, then you'll need a load center that handles that. The typical RV load centers are "right sized" for most smaller RV applications. They come with AC load center and a 12 V distribution panel. Alternatively, you can buy a small AC load center and a stand-alone 12V distribution panel as pictured above.
To answer your questions about interfacing AC devices and DC rated loads: Most AC equipment is rated at lower amperage than a DC counterpart. An amp is an amp whether at 120V or 12V. It is the amount of work that it can do at the different voltages that is important. You can't use a 20 amp 120V device on a DC load of 100 amps. You will soon find out the expense of high amp 12V DC devices.