I've been told that for going to the national parks being there are older and have more smaller lots you sould not go over 35'. I am trying to stay between what you have 31'to 35. As far as the 10 year rule. I've been told as long as you call and make your reservations if they don't ask age then you can stay there at that park. Your Nuwa sounds like it is in really good shape should have no problem. What some of the KOA's do limit how old your f'ver is. I would not try to stay at a KOA, unless they do not ask how old your trailer is. Plus KOA's are more expensive to camp at.
I am sure some of the more seasonal veteran's will chime in with there inputs.
Have you thought of joining the Nuwa owners Forum at www.nuwaowners.org
Check it out if you interested. They have a lot of good folks over there on that forum. Can answer a lot of question along with on Open Roads Forum here.
Hope that helps. Good luck.
Scott
US Navy Retired IC1(SW)USS Fletcher DD-992