There are many guidelines and worksheets that can be found online. Be careful relying on advice, as it is easy to spend money beyond what you actually need. Basic steps:
1. Measure the draw off your batteries per day, and by specific appliances/devices. This is easily done if you have a battery monitor system, run one device at a time and record the draw in amps. You may have to actually pull fuses to avoid duplicating small draws running in the background. Make a spreadsheet/table of the devices and the amps, and add a column with the hours each devices operates over 24 hrs. That yield the Ampere hours (Ah) per day. Total these.
2. Check the Ah of your batteries and what type they are (lithium or lead acid/AGM). So if your Total Ah is 100, and your batteries total 200, that equates to 2 days for lithium. Cut in half if not lithium.
3. Your solar system should be sized to deliver your daily consumption in Ampere hours to the batteries given an assumed number of hours of daylight. I like to use 4 hours of sunlight. So if 24 hr consumption is 100 Ah then solar needs to deliver 25 amps across 4 hours or 20 amps across 5 hours. Then I would double this to allow for a cloudy day.
4. The brain of the system is the controller. Go with good quality, like Victron, and get one with capacity to support expansion later.
5. See what real estate you have on the roof, which can be quite limited on a Class B, and look for panels that can optimally fit. Read fine print as you select panels. Some might show their output in watts per hour, while some may multiply the hourly times and assumed number of sunlight hours (I fell for this with one of the panels I bought).
I built and installed my system myself, and also added ability to turn the system off. See my thread at
https://www.classbforum.com/forums/f...ter-14398.html
Solar panel
Linear Mode
