I don't think there's any doubt that the synthetics can be safely run for longer periods of time, especially with a good filter, but about 13 years ago (and yep, I'm sure lots of things have changed in that time) one of the major RV magazines contacted Chrysler, Ford, and GM to ask whether they approved the use of synthetic motor oil in the engines of motorhomes and pickup trucks used to pull travel trailers. All three said the synthetic motor oils were OK for their vehicles but to change oil on the same frequency as you would with petroleum oil.
The conclusion of their article was that synthetics would be preferable if you lived in areas of extreme hot and cold climates, especially very cold climates, but that they would not be worth the cost otherwise.
And yes, I know a lot of people now disagree and maybe they're right.