Grp 27 wet cell : economical , will work well and charge using the converter you have.
I have used these batts and they worked fine for the 4 years i had them. Two grp 27’s will give you 160 ah and enough cca’s to start your Onan genny .
Correct me if I'm wrong , AFAIK, the Onan’s are usually factory wired off the house bank .
While off grid, these two batts ran my 25,000 btu furnace just fine throughout the winter . I was able to get through the night and the next morning with no issues.
https://www.lowes.com/pd/Deka-12-Volt-800-Amp-Marine-Battery/50183759?cm_mmc=shp-_-c-_-prd-_-sol-_-bng-_-PLA_SOL_146_Riders-Powersports-Chore-_-50183759-_-Online-_-0-_-0&msclkid=1b6d5b9dedea1aa1b46a38ef764b4f8f&gclid=1b6d5b9dedea1aa1b46a38ef764b4f8f&gclsrc=3p.ds
