I leave the side and rear doors open while running. I have the 12v dual pump system from PWS. We have not had a issue with heat while cleaning a roof. My wife runs the pw and keeps everything wet while I treat the roof.
Two ways either mount the compressor outside or you'll need to exhaust the gas muffler outside . I like the sound of exhausting it outside . keeps the compressor out of the elements, security when your working. Forgot who it was on YouTube had a roof vent for his box truck. He also had a microwave if someone knows him. Nice setup . maybe the roof vent with a solar exhaust fan will cure the heat and some fumes sh and fuel. For a house they start around $300.
Good luck. Post a picture
I had a wheelbarrow compressor in my van several years ago and didn't have any issues. A compressor engine isn't usually big enough to cause any issue inside a van as long as the doors are opened up. The heat from the exhaust,..as long as it's not pointed directly at something that is effected by heat,..like the back of a seat,.. would never be a problem inside a van,..the bigger issue is making sure it doesn't stall itself due to exhaust buildup and lack of good intake air. But that isn't likely to happen neither,..just put it into your van and use it.
I had a boss who had a 38 ford f100 and ge had a belt driven air pump in the bed of his truck and designed it to run off the engine to fill it up thru a pulley system he designed don't have a clue about Softwashing with an air pump, but if there was a tank, could it not be filled as needed
While driving to the job and maybe an occasional start hear and there at the job? Or does Softwashing with air require a constant psi