We soundproofed the whole of the inside whilst parked on the road just a couple of weeks ago when it was cold and wet.
You really need an electric paint stripper gun to make the bitumen type soundproofing soft pliable and sticky so it's applied properly.
If it's a sunny day do one side of the van at a time making sure the sun hits the side you're doing as the heat from the sun really does warm the metal up.
You have to do it with the doors open or it will steam up inside the van which you really don't want... if the weather is too bad to have the doors open use the air conditioning to dry out the inside but it does mean doing it with the engine running much of the time.
Good luck and it's actually a nice job doing it and the results are dramatic.