2021 400- wintering 6v batteries

I'd recommend a battery maintainer with a desulfation feature.  This avoids running the converter for months just for a float voltage and provides some battery reconditioning in addition.

This is a newer version of the unit I use.  It can be connected across both batteries (12 volts) as they are installed.  Just disconnect the trailer from shore power.  All 12 volt devices will be operable in the trailer.
https://www.amazon.com/gp/aw/d/B00Q3CM2QY/?_encoding=UTF8&pd_rd_plhdr=t&aaxitk=834da5afd7e052ad3c162381331b35ab&hsa_cr_id=2327324990001&qid=1729116919&sr=1-3-9e67e56a-6f64-441f-a281-df67fc737124&ref_=sbx_be_s_sparkle_mcd_asin_2_img&pd_rd_w=V63Ny&content-id=amzn1.sym.8591358d-1345-4efd-9d50-5bd4e69cd942:amzn1.sym.8591358d-1345-4efd-9d50-5bd4e69cd942&pf_rd_p=8591358d-1345-4efd-9d50-5bd4e69cd942&pf_rd_r=RDDKGDY3GZMS3E3JDEAS&pd_rd_wg=lY8jA&pd_rd_r=42738c99-7a89-4c4f-9068-ab445ce8318e

Here is e pretty good article on the chargers and maintainers with 2024 recommendations.
https://www.autoweek.com/gear/g62502846/best-car-battery-chargers-maintainers/?source=nl&utm_source=nl_aut&utm_medium=email&date=101824&utm_campaign=nl37129530&user_email=205276e166dc37baab0028e9baf3bb217ba1a84a8db495e7324c281e4cf82423&GID=205276e166dc37baab0028e9baf3bb217ba1a84a8db495e7324c281e4cf82423&OO=&utm_term=TEST- NEW TEST - Sending List - AM 180D Clicks, NON AM 90D Opens, Both Subbed Last 30D