Any 4 prong plug to 3 prong 30A plug will automatically only provide 120VAC. I got this and run my Salem 26DBUD on it. You can't provide 240VAC with only 3 prongs, that dogbone is only pulling one phase.
https://www.amazon.com/gp/product/B07MJSX63C/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1