Solar charger for dual batteries

Hello, this is my first post and I hope someone can answer my question. My wife and I just bought an 18' travel trailer so I'm trying to set it up the way I want it. We are always dry camping so battery power is a problem. I'm researching generators and since we don't have an AC or microwave I would like to get a solar charger. (no noise) My problem is I have no idea how many watts I might need. We have a dual battery set-up which lasted 3 days without dying (but almost) over Memorial day wknd. I'm looking at 80 to 120 watt suitcase style chargers and I'm curious if anyone has any info on how many watts are necessary to keep two batteries charged. Thank you in advance for your time and help, Mike