The depreciation on new RVs is quite steep, which is one good reason to at least consider buying used.
I think the advice to avoid buying new is especially aimed at people who are new to RVing and, as such, don't really understand yet what they want and need. Some requirements and wants are rather obvious and straightforward (for instance, if you're camping with your family, you need enough sleeping spots for everyone), but many things are only discovered by experience, no matter how thorough one may be in initial research. It's far more sensible from a financial point of view to gain that experience with a less expensive used RV than with a brand new one. Quite possibly, that new one would either end up being sold fairly soon—at a substantial loss—or else you'll end up living with something that isn't anywhere near ideal for your use.