My experience is that unless the 'surrounding area' is rigid you will get bad soundreproduction . . maybe it needs some 1/4" ply behind the speakers well screwed down.

 

speaker up-grade

Am having same problem with our Fiat based Rapido, I've changed the dash speakers but not much better, I was looking at a Hymer the other day which had speakers fitted in pods on the sides of the pull down bed, It looked ok but not sure if the install improved sound quality as previous poster mentioned you will have to make up some MDF surounds for a pair of decent 6x9's
Chris

 

I had a matched pair of door and dash speakers fitted by ICE in Peterborough and after a bit of 'running-in' so far sound brilliant. They had to fit new mdf collars into the door to hold them well and lined the back of speakers with pro-sound padding. of course, a decent head-unit is essential.

I have an option to go back and consider a separate amp plus parcel shelf speakers but tbh I think what i have will do. On the road you can really 'wick' it up and given that I like heavy rock/metal it sounds a treat. One thing though, when static, you can hear very clearly outside the van so ok if you are out in the wild but pretty anti-social on a site!