I believe you are going to find it easier to locate the 5.25" marine speakers than a different adaptor. Stepping up to a larger diameter speaker typically means a deeper speaker, therefore a deeper mount. You can find it all on Crutchfield dot com.
I would avoid an outdoor subwoofer. They are very inefficient at low volumes... you know the rest about volume. The power demands for a subwoofer can be significant. There are some neat models that plug into 110volt outlet and use wireless audio. If you are going to do it, this is the route I would take. Goes from the patio to the cabin to the RV.