Can you mount a solar panel to the top of the trailer at all?
Like zilo says, if you use a small battery and regularly take it below 50-70% it will shorten the life of the battery.
Having said that, it will still last a fair while. If you are only operating the trailer on weekends or only certain days or weeks of the year, the small battery will still last quite a few years.
My 100ah battery in my previous van lasted over 9 years, even though many 'experts' told me it wouldn't.
Having said that a small lifepo4 battery will also do the job for many years and still not be overly expensive. A reasonable 20ah shouldn't be more than $200. Some brands less than half that but obviously get what you pay for.
I bought this many years ago and ran lights and phone charging off it while camping in tents etc. I think something like that would also suit but would need charging every night.
https://www.facebook.com/74411130570...9453325837262/
I think I got ripped but it served my needs. Identical looking items can be found at a fraction of that price on ebay.