Now for the DC subsystems. The chassis system (for the engine and headlights and so forth) is basically unchanged and unmolested, so this is basically just the house 12V system. There is a little more variation here in how things are done, so this is only a rough suggestion of one possible basic layout that is sensible.
The batteries (assuming 12V batteries) are connected in parallel, and the negative goes to the chassis ground. I've shown a shut for an ammeter or battery monitor in this lead to ground, which allows the monitoring of the current going into or out of the battery bank. This is not strictly required, but is quite handy and a simple ammeter is not too expensive. (Note that some of the inexpensive digital panel ammeters cannot measure current bidirectionally, so you're limited to measuring either charge or discharge current, but not both without some sort of a switch. Others will measure current in either direction, perhaps requiring an isolated power supply for the meter.)
Close to the batteries, as close as practical, is a fuse (or circuit breaker) to protect the rest of the system from a catastrophic short circuit. This might be as good a place as any to talk about fuses and circuit breakers. Their basic purpose is to ensure that the wiring after them cannot carry more current than is safe, and thereby prevent a fire due to overheated wiring. The fuse or circuit breaker rating is thus determined by the ampacity of the wire it's protecting, or at least the maximum size of the fuse is. The wire, in turn, is sized based on the required current for the load. There are actually two related considerations for wire size: one is the wire overheating, and the other is the voltage drop through the wire. The first is basically looked up in tables--you need x gauge wire or larger for y current. The latter is also a function of how long the wire is. Voltage drop is not much of a practical concern for 120V wiring in an RV, but it can be quite important for some 12V applications.
At any rate, there's the big fuse. This could well connect to a bus bar arrangement, or to some other junction, with wires connecting off to a few different places.
If you have a large inverter, it should be connected here, possibly with a fuse if the wire gauge going to the converter is smaller than permissible for whatever size big fuse is used. The wires to the converter should be as short as practical.
Another connection goes through an isolator and to the chassis electrical system. The main purpose of this is to allow the alternator to charge the house battery when the engine is running. There are a few different types of isolators of varying levels of sophistication (and cost); perhaps the simplest to understand is a continuous duty, high current capacity solenoid (a relay) that is controlled by a wire connected to something like the ignition switch in the van. Often there's a pushbutton to turn this solenoid on manually as an emergency start mechanism in case the chassis battery is depleted but the house battery is not, a sort of built-in jumper cables. Some designs are a lot more complex, with time delays and voltage sensing on either side and so on, but the basic goal remains the same. There should be another big fuse for this wire at the other end where it connects to the chassis electrical system. On my motorhome, this connection is made at the positive of the chassis battery.
A third connection goes to a solar controller and solar panels (with a fuse or circuit breaker as needed). Others can give much better information on solar system design and sizing than I can.
A fourth connection goes to the DC distribution panel and converter. I forgot to show it in the diagram, but there's quite often a disconnect switch in this connection to enable the house system to be turned off and prevent the battery from being discharged. Usually this is a latching high-current relay, but a plain manual switch with suitable ratings also works. Again, there's a fuse or circuit breaker to protect the wire to the distribution panel.
Particularly if the converter/charger is in the same chassis as the distribution panel, it's often connected in there. The charge current may well be the largest expected current on this connection.
From the DC panel, DC circuits go off in all directions to all sorts of things: ceiling vent fans, lights, the fridge, the furnace, the water pump, the water heater controls, 12V sockets, a radio, that 12V gizmo your great aunt Sally sent you, and so forth.
