The reason the speaker sounds better with a backer box is you are trapping the sound waves and containing the back vibrations to within that box. Without a backer box, you essentially have an infinite baffle. That also works, but it requires larger drivers and a lot more power.
Unfortunately in a RV I doubt you have much room for either larger drivers or a backer box. The best you can hope for is to use as large a driver (heavy magnets are good) as you can, with as powerful an amplifier as you can. There are limits of course to the amount of power you can draw in a RV.
The stereo unit that comes with most trailers is a double din car stereo. You can easily replace that with one that has good pre-outs. Then add a nice car multichannel stereo amp. It doesn't (probably shouldn't) be very powerful. It just needs to produce clean power.
Then depending on whether you want to use the 12V system in the RV or the AC outlets, you can go with a small subwoofer. You can even use one of those prebuilt sub boxes with a driver like the ones in a pickup truck. If you want a home subwoofer, there are any number of units out there that'll do just fine.
The key is you want your small speakers to just reproduce the mid and high notes. Filter out the low note signals if you have to with a simple in-line filter. That will allow those small speakers to do what they do best. Pass the low signal to the subwoofer and filter out the lowest part of the spectrum, maybe everything below 80 -100 Hz. If you're in a RV park your neighbors won't want to share your music. Low bass waves are long waves. Long waves penetrate walls and travel further.
It would be good to have a equalizer of some sort. Some of the better car stereo head units will have them built in. That way if you're using a car audio subwoofer, you can use the built in equalizer to play around with the response.
