I’ve just come back from a test run I’m the new van after sticking solely coat to the sides and roof. I put on about 80/90% coverage . Well it worked! Not going to bother going further than the silent coat stage. Definitely kills the rumble. I think carpeting the panels would sort the brightness reflective sound inside but I can live with that for a bit. If the speakers and subs went in then I think I would add in felt behind the panels plus carpeting .
its all on your budget , what you do to your van but felt would you not fit sound deadening foam there's a whole range of products for deadening turning your van into a speaker Box for your Sub or do you want people to hear you coming lol. if you dont want to Thermo insulate your van is up to you it helps keep the van cool as well as warm.