RVs are not made like yachts. If I lean on the wrong wall in some travel trailers, I have a good chance of damaging it. If I run the water heater empty, I now have to buy a new WH. If I run the fridge on an incline, that's a new fridge. If I leave a window open during a storm, that could render the rig to scrap with water damage.
Then, there are critters like bedbugs which people tend to bring with them.
Insurance? The problem may not be caught at once, but later on, and there would be 0% chance of a claim being paid. That 1 million dollar insurance policy is actually a tad puny. I can see some people hitching a 7500 pound dry weight trailer to their small SUV rated for 3500 pounds, using a one inch to two inch hitch receiver adapter so the class IV trailer hitch works on their class II receiver. Then, the renters get drunk, get in a wreck, hurt a number of people, and all of them sue the owner of the RV because it jack-knifed across the road and caused a chain collision.
Too many things can go wrong, and the cash coming in isn't enough to justify it. Especially with most RVs that are made as cheaply as they are. A boat is intended to last for decades. A newer travel trailer tends to convert itself into component pieces in less than 10 years.