“Saying and doing are not the same”; exactly right.
Have you thought about finding a qualified driving instructor that is a good fit for you both. Your partner and you can learn what it’s like while gaining instruction in a likely stress free pace. Without making a huge financial and emotional commitment first.
You could rent a similar sized rig but some instructors also RV and may have one for you to drive. Also think about the floor plan.
As to “ how much does length impact the ability to drive?”
There are too many variables. Have either of you driven large vehicles? Are either of you uncomfortable driving a personal vehicle? Are either of you apprehensive of the others driving now? Are you both good with backing up?
At first, I had trouble staying centered in the lane even though I drove large fire trucks for years. Through an instructor, I found the problem was the coach actually steered too easily, I was used to a “truck like steering feel. It is more like a luxury sedan when the rig is sound, balanced and set up correctly.
My issue was fixed by changing how I held the steering wheel. From the 10 and 2 way I learned (and taught) years ago to today’s air bag position 9 and 3. (Also recommended by NHTSA
) This allowed less see-sawing, more relaxed holding of the wheel. Using the arm rests. Less tiring overall.
But each person is different. And I had the other spatial awareness of large vehicles in my past experience.
Get a good instructor and as you already know, “do the doing” part first.
Good that you are thinking ahead.
While this is my opinion others are also correct in how they respond.