First, I can't imagine where you heard that Winnebago is the safest. 'Safe' depends on the driver and other drivers.
As far as fitting a 40' into state parks, it depends on what parks. Since you mentioned Florida or the east coast, I would suggest picking out a few parks that might interest you and see what their camp site sizes are. There are no two parks that are equal. For private RV parks you will have no problem.
We traveled mainly in the West and had absolutely no problem with public parks - except for California's state parks. However, there are always options and we found wonderful California national forest campgrounds that we did fit. You just have to do your homework.
As far as driving a 40' motorhome....they are very easy to handle. Both of us drove it, including in the mountains.
You mentioned cul-de-sacs....for that there would be many differences in size - some have very little turning space. Personally, we wouldn't get ourselves in a situation of cul-de-sacs.
I would get the RV that you would all be comfortable living. Bunks would be great for you. Good luck!