Woody,,
At least one company may make a single battery* to replace your eight, but you didn’t say what size batteries you have, their voltage, and if in series/parallel. More importantly, you need to know how many amp-hours you have, and how many AH you need to run all your gear & appliances. A pair of 6V in series do not add their AH together. Pairs of batteries do add their AH. However, keep in mind that lead-acid batteries give you only 50% of use. I’m my case I had four 250AH deep cycle 6V batteries in series then parallel, so the two pair were rated at 500AH. At 50% SOC, that left me with 250 useable AH. I replaced them with three Battle Born 100AH Lifepo4 batteries, giving me 300AH—all useable. These and a Victron 3000 Inverter/Charger keep my RR running well through the night.
*One vendor showed me their 300AH Lithium battery costing $5995!