Interesting discussion..
It won't hurt the water heater to leave it on when you're camping (or parked in the driveway for that matter). Then the discussion takes a turn,
If you leave it on, you have the convenience of hot water when you want it without waiting until it heats up, but you are consuming propane/electricity, whether it is more than running it only when needed vs leaving it on is yet another discussion.
I would say it is personal preference, I have done both and will continue to not stress about it and do it which ever way I want for that trip...
I would further suggest to those that are saying we're wasting power or propane, I suggest you also use the same logic for your home. there is more potential for savings if your home water heater is managed. I had to replace my water heater elements on my home water heater yesterday and was shopping online to see what is available should we decide to replace the whole unit. They have smart water heaters now with logic so you can have it learn your usage habits and heat water accordingly, it also standard mode and vacation mode..