Unfortunately, water intrusion along the belt-line is not an uncommon problem. The challenge for making any repair is that the sub-floor is a sandwich construction of two outside layers of OSB and a fairly thick inter-layer of styro-foam. You may have to cut the OSB back to a structural support, then re-bridge the space with independent layers of OSB and styro-foam. As mentioned, if one of the OSB layers and the stryo-foam are still intact, you may be able just replace or repair[git-rot????] the affected OSB layer.
2003 40' MDTS
Garden Ridge, Texas