We started out with one 50w panel attached to the roof of our
Scamp 5er, but found (during a cold, cold September night in Yellowstone National Park) that 50w is not enough when your
furnace runs a lot. We added a second panel. In all, we spent around $500 on panels for the Scamp.
We have a second, project trailer we're working on, and I just bought a 100 watt "polycrystalline"
solar panel for it for less than $140, delivered. I haven't installed it yet, but I tested and found it delivers pretty good power levels, even at steeper angles of the sun and across a pretty wide temperature range. (Monocrystaline and polycrystaline panel power production falls off rapidly once they reach 120 degrees or so.) On the hottest day of the year here in Vancouver, WA I tucked my panel into its box so air could not circulate underneath and cool it, let it heat up in the sun to the point where I literally burned my arm when it brushed its aluminium frame when I picked it up after testing, and it still managed to produce 23.4 watts of power with the sun hitting it at a 30-degree angle.
Back to how large a capacity panels do you need. If you're like me, which means you use all LEDs in your trailer, have a
furnace that gets a fair bit of use, and frequently charge camera, laptop, and cell phone batteries, 100 watts of roof-mounted
solar is about right. Take away the small electronics charging and change the heater to Wave 3 a catalytic, direct vent heater that uses no
battery power at all, and 30 watts of detached
solar panels that can be placed in sun away from your trailer should do the job. Where you fit on that spectrum is up to you.