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.