Why don`t you mount your machine on heavy duty runners/rails and bolt it to the floor of your vehicle, then you can pull out pressure washer from the back of the van without physically lifting it, .......make you sure can lock it place though when you ready to go to your next job via pins!.
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See if you can buy `foil insulation` for industrial piping,....its a type of wadding encased in foil,. then wrap this around exhaust pipe and tie with plastic cable ties, then just hook the pipe on your door or roof gutter instead of drilling holes in your roof,...make sure the pipe is long enough ( a few feet above your head for dispersion).
Your washer should not melt any tarmac,...you need to fit plate between the underneath of your washer and the floor, plus buy feet for the frame to stand on!