OP Welcome to the Forum
So far you received a lot of good information from your original question.
Regarding the RVIA seal, they don't mean anything to me, they are an industry standard that works for the Mfgs, not the end consumer. Useless in my experience and opinion.
Also from experience, there is no such thing as a non-negotiable price. This is even more true when buying an RV during the winter months. RV's in the U.S. all have high asking prices, new or used, that are falsely set based often on the inability to find accurate price figures like those that are readily available in the U.S. auto industry. Negotiate.
A generator may be your best friend when traveling around the U.S. You can stop and park overnight, and in the Western states multiple nights for free and still have an electrical connection to run your lights, fridge, TV, keep the batteries charged, and anything run by electricity. Look up the ongoing thread on this Forum talking about parking overnight in Walmart parking lots. RV'ers here do this at big box stores, church parking lots, and restaurants that encourage it (Cracker Barrel).
As far as size comments go, one of the first things I told my wife when we were shopping for our first RV 17 years ago was "Honey, it's not always nice outside when camping".You will be starting your journey in January which means cold wet weather in many parts of the U.S., even the normal warm weather areas of the country such as Southern states, or the American West/Southwest. Going up to a 27-28 ft Class C would make a huge difference.
Don't worry about where you can go with a 24-31 ft Class C in the U.S. Remember, everything in the U.S. is huge compared to Europe. Our roads are spacious, and our parks, are places to see and go to. E Our campgrounds are huge, especially if they are operated by a government agency such as county parks, state parks, national parks, and C.O.E.(Army Corps of Engineers) campgrounds. I have watched a couple of RV YouTuber videos (Traveling Robert and Endless Adventure) of camper vanning in Europe and yikes some of the roads are narrow. You could drive a 30 ft C right through NY City or LA.
Keep in touch with The Forum now, and later when your adventures begin.