Well that's a loaded question...
I'll let others dive into the usual argument over which solar setup is better
For charging phones alone a solar setup is almost overkill. Phone batteries are small compared to the trailer battery and won't drain a lot from it. I mostly charge my phone in the tow vehicle when I'm going somewhere.
There are other power uses in the trailer though, the fridge control, lights, water pump, fans, furnace etc. so if you want to stay out for more than 2-3 days you need a way to recharge the battery.
Personally I use a tiny 40W setup with a Wolf Pup 16 FB (older version of the 16 FQ). It works ok when there's plenty of sun, but in sites where there's only 3-4 hr usable of sun during the day it does not keep the battery topped off.