I can imagine the argument is that unless you're periodically getting tested to you have a likelihood of spreading if asymptomatic over a broader geographic area while traveling.

That said, I personally think now is a great time to get out into nature and travel more locally pending the regulations in your area(s).

My van had a back-end that was size approprate to fit a YETI brand cooler (Tundra 45) between the raised platform in the back of the van and the seat backs so I could get in and out of it easily while having it to write on or use my laptop computer on (it doubled as a desk when I felt like sitting indian style on the floor of my van). This was essential and I could rely on fresh fruit all the time kept in the back of my van along with well various other fresh produce.

All of those things reduce risk, they do not remove it. And given that you can be asymptomatic for two weeks there’s a very real danger of spreading the virus. And there isn’t really a good reason to be doing it right now other than “I want to” so it’s not difficult to see it as a selfish act.

(the friend travelling from city to city and staying in Airbnbs sounds considerably worse to me, though)

Probably not that bad since, with the advent of masks and social distancing, supposedly ~95% of cases are NOT from community spread. Most of the spread is from dense living situations and meeting with friends and family without masks when you shouldn't.

It’s no more selfish than those going to pickup takeout food from different restaurants multiple times a week. If you’re going to chide a van camper, chide every single person you see outside not having all of their groceries and supplies delivered to their doorstep.