Polk Audio makes some shallow mountable speakers that are a major improvement.
Here's an old thread..
http://www.jaycoowners.com/forums/f1...kers-8092.html
I'm adding a Sub to mine. The factory head end on ours is decent, it has an unused sub woofer control line coming out the back. We aren't going for volume, just quality. Sub will be self powered so I don't have to worry about destroying my battery when we boondock (will just turn it off). I have an Alpine 12 inch sub, boxed, with an MTX amp on the side I was considering installing in the camper as it's just collecting dust in my basement.
I have to be careful that I don't turn into 'that guy' when camping though so it's probably not going in. I think the Polk Audio's are a good upgrade.
Here's a more recent thread, those Polk Audio's may be discontinued by now, but there are other options.
http://www.jaycoowners.com/forums/f5...ade-42468.html
Best thing you can do is just pull one of them and take it into best buy, or at least have something to measure your replacements against.