IF you have the patience, time and can lower the charge rate from 40 A to 30A or 32A, it should not "blow" anything up.
But LARGE RVs when the weather is warm, and twin A/C units are running, etc. probably put more stress on the same electrical system: Dodgy wiring will fail for those RVs also.
Some of the NEMA 14-50 outlets are NOT wired with 240 V power (~28 miles per hour), but instead 208 V, and they are slower (charging) anyway, 23 to 26 miles per hour.
Many "50 amp" RV's are actually 120V and simply use only 120V from neutral to the two hot legs. In fact, one campground that Liz and I stopped at had only one functioning 14-50 outlet for charging Teslas, because the campground "electrician" (I use that term loosely here) wired both L1 and L2 to the same leg of service in all the rest of them. Most RV's will be able to work using this, because they only need 120V. When measuring line-to-line, you get 0V because it's the same leg and the Tesla can't use that.
Cosmacelf is right when he says they're dodgy... so many pedestals are out there with very old aluminum wiring that has seen its day... broken neutrals creating a return path via ground, etc. I've encountered my share of "box hopping" to find an outlet that seemed reliable enough.
The average "50 amp" RV doesn't draw anything near what a Tesla draws. Even the largest Prevost coaches I've seen can run full blast on a 10 kVA genset-that's 42 amps with a perfect PF, and it's not a continuous load like the Tesla.
I stopped at a KOA near Maramec Caverns in Missouri, who told me that she would have to charge me $25 for charging my car, whether or not I paid to stay in a cabin, day or night, empty park or full. I just shook my head and tried to educate the owner, but she was adamant. Meanwhile, down the street at the Maramec Campground, pay $5 day use fee, use the playground, beach, barbecue grills, etc., and get a full 40A charge.
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Gear, what I was trying to point out is the perspective of some RV park owners. They see someone driving a $80K car negotiating over $10, and their reaction is that this car owner must really think I'm some low class person who would beg to make $10 revenue today. Whether you realize it or not, you really have disrespected the RV park owner. Now some RV park owners are secure enough in their own skin to brush off such unintended slights, but not everyone.
If someone called me a fool for refusing to pay $25 for a charge because I drive a Tesla, I'd tell them what my (very poor) grandmother used to say... "pennies make dollars", and that it's that attitude that gets someone in a position to purchase a Tesla in the first place. I'm willing to pay a fair price. I won't haggle for an hour to get him down $2, but I also refuse to pay $25 for the charge unless it's an emergency.
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$5 isn't even worth the time it takes to answer the phone (just slightly exaggerating). Maybe because I've run small businesses before, I'm kinda sensitive to this. People seem to have no idea how expensive it is to run a business.
If your infrastructure is sitting empty, it's making you nothing. $10 revenue is better than $0 revenue. Yes, it's expensive to run a business, but refusing business is one way to quickly run out of business.
I will agree with you that if things are crazy busy and you're having to spend 15-20 minutes discussing electric car charging and blah de blah de blah, it's not worth the time to have the conversation, or if the park is almost full and it's a choice between leaving a spot open for the nightly $50 rate for an RV pulling in vs. $10 from a car owner sucking juice for a couple of hours.. But if it's a simple conversation with plenty of spaces left - "I have an electric car, how much to park at a 50A outlet for 6 hours?" "I don't know what it would cost." "About $5 at commercial rates (coal country), how about I offer $10?" - then why not?
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I'm going to be charging at an RV park for the first time this summer. The KOA I called was at first somewhat reluctant, but then agreed as long as I paid the daily rate ($60). I agreed because without this location my trip will be completely derailed. I will be sleeping in the car, and they have public facilities (bathroom, etc.) For me it's worth it. If I was driving cross country this way then $60/charge would be objectionable.
At the campground we stayed at, they graciously let us use a 240V outlet for free by paying the cabin rental (in fact also offered to let us pull our cars behind the fence at the cabin and run a cord out from the dryer socket if we needed to).
At one time the KOA office had recommended to their owners to charge $10 for day use of an outlet if the grounds weren't full, but I think they backed away from that, at least according to a couple of KOA's that I called.