Info from a tire expert with 40 years in the industry would agree with your tire guy. He bases his opinion on the fact that tires are much better made than they used to be, the weight of these vehicles in the rear doesn't make it much of an issue.
In fact, here is a quote taken from a different conversation on the same topic.
"Tires are checked for force variance before they ever leave the plant. The balancing is only necessary, in most cases, to compensate for possible issues when the tire and wheel are mated. At most, that assembly will only require a few ounces of weight. A rear dual assembly is so heavy that a few ounces of weight variance isn't going to be noticeable. Think of a fly on the back of an elephant. It won't hurt to balance them but it won't help much either."
And to finish that off, "Oh, by the way, since you are new here, I retired from the tire industry after 40 years. I have been involved in the sales and servicing of over 10 million truck tires, have worked with some of the largest trucking fleets in the country and have owned a motorhome for 10 years so I probably know a little more than some guy who has only been in the industry for 10 years and may never have sold a motorhome owner tires."
Previous quotes were used by permission. I have a lot of respect for the author, so I am just putting out what I believe to be factual real world experience. You are of course free to take any advice you wish.