Some OEM car speakers have plastic enclosures around the bottom side of the speaker...particularly door speakers.
Crutchfield has something similar in these:
Crutchfield clickyDepending on the speakers you chose, if you want any real bass response, you'd have to "port" the enclosure to allow air to move and thus not restrict the larger woofer diaphragm from moving as much (yes I know, air is a compressible medium...but if it doesn't have a place to go, it will restrict the woofer somewhat). You would also need to puncture the enclosure to run wires, though that can be sealed up with caulk. I'd recommend either a small hole or even better a semi circular "C" type of cut on the surface that would cause the most torturous path for water to make it inside the TC (probably the top surface for a camper, but bottom surface for a car door application). Either way, I'd still go with marine grade speakers as they will surely spend some time getting wet when driving through a rainstorm or washing the TC.
And, FWIW, my outfitter has 2 marine grade outside speakers without an enclosure. I've had the camper since Dec 2007 and have had water enter from a few areas, but not the speakers. They are set up with the car stereo deck I have in the camper such that they are considered the "rear" speakers. 99% of the time, I have the fade all the way forward so that only the inside speakers transmit audio. The rare occasions I do use the outside speakers, I'd say 70% of the time I'm not in a campground (read not near anyone else), 20% I'm in a NF style CG with very few people around, and the remaining 10% of that original 1% usage....I'd rather listen to my own music than some fool who thinks the entire CG wants to listen to his music. Even in those cases, I'm never loud enough that you can hear my music more than 10 feet away as I'm likely outside right next to where the speakers are wrenching on a bike.
But, the $5 dollar question is: Would I cut holes in my TC if I didn't already have external speakers? Nope. Not worth the hassle to me.