Maybe it's more practical to get a second one that is portable. We have a normal RV AC, rooftop installed, but haven't had this issue yet. At home, we have a portable AC, and it really packs a punch when it comes to cooling too. We always said that if we were to need the extra cooling power, we'd get the one from home on our trips. It's more practical and it will definitely cost you less overall than installing another rooftop unit.
https://www.optimainstitute.com/ac/small-portable.htmlhttps://www.tripsavvy.com/rv-anywhere-without-air-conditioning-2912341