There are some here who have gone into solar having no clue how much power they use, or how little they can get by with. You don't have to be one of those. With a multimeter that has a 10 amp or higher DC measuring capability, you can get an idea.
To get started, unplug from shorepower, turn everything off in the trailer (don't forget the antenna amplifier) and disconnect the cables from the battery negative posts. Put the black meter lead on one battery's negative terminal and the positive (red) meter lead on the cable that went to that terminal.
Don't be surprised if you see current being drawn. This is the sum of your phantom loads, the LP, smoke, and CO detectors, the radio remembering its settings, and any control circuit boards in appliances. This is your base load in amps and you can multiply it times 24 hours to get daily base amp-hours. Amperage (aka amps) is an instantaneous measure of current, amp-hours is the current used over time.
Now turn on one thing at a time, note the meter reading, and subtract the base load to get the amperage that thing draws. Multiply that times the number of hours per 24 hour period you think you will use that thing. Go through the trailer making an electrical use inventory. If you have Fantastic Vents, check them on each speed.
Don't forget to do this with the refrigerator and the water heater. Although they run on gas, they have circuit boards and ignitors, and possibly an electric gas valve. These cycle on and off and you need to measure their draw in each condition. You'll have to estimate how long each will be on vs off over 24 hours to calculate the total use for the day. Personally, I turn our water heater off until about 1/2 hour before it's needed.
I do NOT recommend doing this measurement with an inverter or the furnace fan on. Even if they aren't rated as high as the multimeter's maximum DC current, they have a start-up surge that may exceed a 10 amp DC meter's rating. For the furnace, you can use the manufacturer's rating.
For the inverter, you can use the wattage rating of the 120VAC appliance it will power, divided by 10 (that accounts for less than 100% efficiency) to estimate the instantaneous amperage the inverter will draw from the batteries. Note that the power drawn by an "X" cooking power microwave can be 1.5 times "X". Look at the data plate.
A total instantaneous amperage draw in excess of the sum of your batteries' 20 hour rate requires special handling. For example, a 100 amp-hour 12 volt
battery will last from full charge to a defined full discharge 20 hours at the 100/20 = 5 amp rate (or two in parallel, this is a 10 amp rate).
But this does NOT mean it will last 100 amp-hours/25 amps = 4 hours at a 25 amp discharge. At that rate, which is the one used to establish the Reserve Capacity rating, the battery will only last roughly 180 minutes or 3 hours. One way of looking at this is that a battery that's rated 100 amp-hours at a 5 amp draw is only a 75 amp-hour battery at a 25 amp draw. Another way of looking at it is that a 25 amp draw is like a 1.33 x 25 = 33 amp draw when it comes to calculating use out of 100 amp-hours. This is due to something called the Peukert Effect. I won't go into the calculations, but will say that the higher the draw, the worse this effect is.
Two paralleled 12 volt
batteries, each supplying 25 amps, is about what it takes to power a little 500 watt inverter supplying its rated 120VAC output. A 2000 watt inverter can draw as much current as an automobile starter and many know how quickly that can drain a battery.
I am not a fan of inverters, but there are some things that need them. Such things are laptop computers and LCD TVs for which there are no 12VDC adapters, satellite receivers, etc. Fortunately, these are low draw items. On the high draw side, I'll include vacuum cleaners. I've never found a 12 volt
vacuum that works worth a darn. Just minimize the time you use a 120VAC model on an inverter, or better yet on hard-surface floors, use a dust mop or broom.
The worst thing one can use battery power for is making heat. If you do the math, the effective amperage drawn for a coffeemaker or toaster is mighty high when the Peukert Effect is considered. Fortunately, a pop-up toaster isn't on very long, but I prefer to use a camping toaster over a gas burner.
I'd rather you use an inverter to power an electric coffeemaker than power up a generator right next to me early in the morning. If you're smart, you'll shut the inverter off as soon as the coffee is done and pour it into an air pot thermos. But if you're smarter, you'll boil water for coffee on the stove and pour it into a Melitta or Dripolator, or use the Coleman drip coffeemaker on the gas stove.
When taking inventory of electrical use, you can see how much these high draw of inverter-fed applications increase your expensive solar requirement. You may also be surprised at how much long-duration, low draw things use over a day. Over 24 hours, Fantastic Vents can use much if not all of a large panel's daily output.
Once you've done your inventory, consider the rule of thumb that a horizontally mounted solar panel typically provides 1/4 of its wattage rating as amp-hours per sunny day in the summer. To account for cloudy or rainy days, you may want some reserve capacity, or you may just choose to carry a small generator to augment your solar setup.
By doing these measurements, you can make an informed choice about how much capacity you want AND how much value conserving electricity has for you.