Managing a retail store will most certainly be busier than an RV park and you're right, a customer in front of you would be more important. But.....how many customers did you lose by not having someone else answer the phone.
I've been to a LOT of RV parks over 40 years and most of the small mom and pop campgrounds just leave the office and do their thing. They show up later and hopefully return calls. This was fine 5 or 10 year ago, but customer service is more important than ever and technology can allow you to roam around without being tethered to an office phone.
Whenever I can, I try not to do business with companies that are too busy to take my call.