Very good, I am familiar with the marine fan options and have never seen any that fit in the standard RV opening. Thought you were suggesting something specific that was available for a simple replacement.Marine fans are typically made to "hatch" sizes or require custom installation. Without knowing the size of the current opening, any specific product recommendations would be based purely on speculation. I do not know of a "take out a MaxxFan #245 and install Camframo Fan #594" type of solution, and would be surprised if there was one.
That said, check with Defender Marine, West Marine, Fisheries Supply or a local Chandlery, in the "ventilation" section of their respective website or store and look for hatches and fans made by Vetus, Camframo, Lewmar or other quality manufacturers.
Spending some time in sailboat forums will net you a world of knowledge, as ventilation is crucial on a sailboat, especially in common cruising grounds such as Florida and the Caribbean. Remember, sailboats, even small ones, typically have multiple hatches to keep air moving.