Lots of people have mounted batteries where the spare tire is located and can comment on that specific location. I mounted four Battery Born 100Ahr batteries under the slider doorway step. They are mounted on their side to stay above frame rails. This location is closer to van alternator and driver seat for electrical connections and has worked out very well. I monitor battery, outside and exhaust temps and the batteries have have always stayed well within their operating temp range with ambient temps from -5 to 112F. From my experience leading battery cell manufacturing engineering at Tesla, these batteries will outperform lead acid batteries outside and is a fine option to save space inside as well keep CG lower.
Have owned 2007 Sprinter SMB and have now converted a 2016 170" 4x4 w/custom interior made of aluminum and HDPE, high powered electric system, hot water, dual air compressors and systems, electrically raising queen bed, large bike garage, rotating two person bench seat, custom rack, suspension improvements, skid plates, lights all around, custom roof rack, and many other things. Now considering a 2021 E450 converted to 4x4 with custom box.