If you nearly always camp under tree canopies without direct solar access then its probably not worth bothering with solar, PV modules just don't perform well under these conditions. Even partial shade causes a severe reduction in performance because the cells in the modules are in series so shading one cell pretty much blocks the output of the whole module. Then you also have the issue of how to maintain battery charge over a series of cloudy days.
in this scenario just get a generator and run the battery charger with it for a couple hours a day (depending on your usage) instead. Generator type and size will depend on whether you want to run your air conditioner and/or microwave directly from it which are the loads in your trailer which require and AC power source. You can switch to a 12Vdc TV set pretty cheaply and easily if you use that a lot.
If you do mostly camp in direct sun and in sunny weather then by all means go for the solar installation. Keep the genny for cloudy day backup and for when you are camped under trees.
I'd suggest you try the dual battery installation first and determine how long that lasts for you before deciding how large a solar array you need. If you install say a dual 210 amphour battery bank and it gets down to one light on your led display after about 48 hours then you would roughly need about 200 watts of PV (unshaded, summer conditions). Shut off all your loads and charging sources and wait half an hour before checking the display. If you get down to the same state of battery charge after one day then you'd need double that and half that if it takes 4 days.
Double these estimates for late fall/winter conditions (less sun in winter of course). Also, your electrical loads will be different in cooler weather (more lighting, heater usage, probably more entertainment usage, less fan usage).
This gets technical but you could also try to estimate your daily electrical load. In case anyone is interested, here are measured current draws from my rpod:
single led lite | | 0.2 |
dual led lite | | 0.4 |
bath lite | | 0.3 |
outside lite | | 1.4 |
small 12V fan | | 0.5 |
fantastic fan h | | 2.8 |
fantastic fan m | | 1.9 |
fantastic fan l | | 1.3 |
fridge 12V (don't use except when driving!) | | 10.7 |
water pump | | 5.0 |
TV 12V (I replaced the 120Vac TV with one of these, easy mod) | | 1.5 |
Audio unit | | 0.4 |
heater (when on only) | | 2.0 |
water heater (when on only) | | 0.6 |
You would need to estimate how many hours (or fractions of an hour) each device was operating, multiply by the current draws, and add to get a 24 hour total. As a shortcut, if you take that calculated amphour total and multiply by 3 that is roughly the size of solar array you'd need for summer unshaded conditions. For example, I use about 75 amphours/day in the summer so I'd need about 225 watts.
Note that I am not including the microwave and air conditioner here. These use a LOT of power and would require a large DC to AC inverter to run. You could consider this for the microwave if it got light usage but just use the generator for the air conditioner if you have to run it when not on the grid. There are posts on generator requirements and air con mods you can study if you want to do that.
For larger scale solar applications, say above 200 watts or so, consider buying one or more residential style PV modules as they are far less expensive than the RV kits. If you have light electrical loads and can get by with one of the portable RV kits then great, do that.
These larger modules operate at higher voltages than the smaller (nominally 12V) modules. You can get MPPT type charge controllers to optimize power extracted from the module and to step down the module voltage to charge your 12V battery bank. You will need a charge controller regardless of which module type you get anyway.
I haven't yet added solar to my rig as I just got it this spring and I want to assess my fall and winter usage first, so I don't have any specific recommendations for solar installation hardware yet, but larger modules/systems are generally roof mounted, they are really too big and heavy to be considered very portable. In the meantime I just use my 2kw generator, and will continue to carry it after I install solar for use when I am camped in the shade or run into prolonged periods of cloudy weather.