Great question and I'm sure you'll get different answers but I'll weigh-in first.
I get a lot of first-time renters with little to no experience. There's so much that can overwhelm and intimidate them. The last thing they need is to be responsible for filling up propane which adds further complexity to both my return policies and potential for backing into a pole or other object (which gas stations usually employ to protect their pumps).
On top of that, especially during the summer, one tank can last for months so I just fill up in between a trip to the self-serve wash, before one of my own camping trips or coming back from the shop for routine maintenance.
My thoughts are, let the renters enjoy their trip without too much hassle and work LP refills into your RV maintenance cycle. Just make sure you build that into your yearly maintenance costs and charge accordingly in your daily rate.
The Leamon Party Bus