My mom and stepdad rented a camper van via Outdoorsy last summer. Their experience was good, but be sure to read the fine print. For example, after 150 miles a day, they were charged mileage, which isn't uncommon, and they did know about this before they rented it so it was not a surprise. But I could see some people not paying attention to that. This doesn't appear to be a thing with trailers, but definitely with vans/motorhomes.
The unit itself was nice, well maintained and clean. The only issue they ran into was the space heater in the inventory list was missing, and the owner didn't catch it before they picked it up. Apparently the prior renters had kept it. So the owner offered to reimburse them if they went to buy one themselves.
In the end I think it ends up being similar to Air BnB. It's going to depend on the owner and how well they clean/maintain the units. You can read reviews of each unit to determine how nice it is.