From this post and your others, sounds like you are doing your research and really thinking this through. I believe you will end up with a good choice for your family.
In my opinion, 3 to 5 years old seems to be the sweet spot, someone else has taken the big initial depreciation hit. And the rig is still pretty new.
But some others thoughts you may consider.
depreciation only counts if you sell it
The concept of usage, if you use it, enjoy it, you will more than get your money out of it.
We bought 4 years old 10 years ago. Paid 31K or 32K I think, probably with repairs , tires etc now in about 40K. Now worth maybe 16 or 17K. But the things we have done the places we have gone, well worth that 23K, or so.
Buy it once, buy it right, get what will work for you long term. If you buy it wrong and trade in a few years that will really kill you.
And lastly in my opinion, don't buy so much rig that it is all you can afford to do. ( no other non motor home trips, redlining the budget)
Get something you can easily afford, and you don't feel like you have to use it all the time.
As I said, we have had ours 10 years now, will probably keep it another 15. By that time. it will be worth about 2 bucks. But in my opinion, it will have been money well spent