HI
Solar is part of a balanced system of knowing your needs, conserving where you can without sacrifice, having enough battery to run those needs for a few days when the sun doesn't co-operate and a back up means of charging when the sun hides too long.
Figuring out your needs can be done a number of ways. Your equipment will have a plate showing it's draw. You can use a $20 Killawatt to test each piece. You can use a amp counting meter to record power in and out of the batteries. Me, I just took a resting voltage of the battery every morning for a few seasons for a rough estimate. Once you know how much you use you can know how battery to get. don't forget the little things like lighting.
Conserving power is easier then replacing it. LED lights to replace the incandescent bulbs in your van can be had for less than $2 and will greatly reduce the drain on the battery. Converting from 12v to 120v to run a TV is inefficient, there are TVs that run off of 12v or if they have a power brick, you can replace it with a 12v cord. The best I've seen was a 26 inch on 12v pulling 26w.
So if you use say 40 Ah a day, and want a few days of buffer for bad conditions, you need 120 Ah of usable battery. A battery should never be taken down past 50% (12v) so you really need a 240 Ah battery to go three days. Unless you have room for a massive 8-D, you will need to combine a pair of batteries to get enough. That means two 6 volt in series or two 12v in parallel. Room is a limitation, so that has to be considered too. After all of that there are still choices depending on your situation in type of battery.
Now that you know what and how much battery, you can pick out your solar. A watt per Ah of battery is a good start, I prefer two watts per Ah because when the sun does finally show again, I want enough power to push the battery around. I can take my system down to 50% and in one good day bring it back up for another 3 day run without sun. Like the batteries, solar is limited to how much room you have to mount it.
When choosing a charge controller make sure it doesn't just fit the panels but also has the ability to work at the voltages specified for the battery. Some controllers are not adjustable or produce voltages high enough to charge certain types of batteries.
For back up charging we use a generator or if that's not available, a set of jumper cables between the truck and trailer. Being in a van means that you can use the van itself as a back up. You can isolate the cabin battery from the starter battery and the van will charge just one. A small inverter generator would use much less gas but you will need either a converter in the van or take a auto charger with you.
I call this the ever tightening circle because every decision you make will affect multiple components. Don't buy anything until you have a handle on what you need or you may find it a huge waste of money.
