First, don't stress so much over it. It really isn't as hard as people sometimes make it out to be.
After that, the second most important thing to remember is slow down and take your time. This means in campgrounds too. A lot of people worry about their first time pulling in. Don't. We've all been there. No one's laughing at you.
My first time, some guy ran over and said he walk me through the backing in part. Great guy. Now, I'm the guy that will recognize when someone is nervous and run to help. And I'm not always the only one.
Don't be afraid to tell the desk when you're checking in. Most times, they'll have someone go with you. Not only are they normally very friendly, but they don't want you slamming into the power pedestals either.
Even guys like me who've done it a thousand times screw up. Don't be afraid to start over. Even if it means driving around and starting over.
As far as what to buy, we'll that's up to you. Once you have more experience, a larger unit is not much of a problem in smaller campgrounds. Some of our state and federal campgrounds do have length limits. Usually due to the hard turns getting in. Where I live, the federal campgrounds could easily fit a 38 ft and truck. Nice long spots with tons of space. Roads getting in are a different story if you're new. Only advice there is call and ask. Campground hosts and ranger stations are great sources for info.
I buy used. I feel I get more value for my money. I don't worry about warranties. But you have to know going in that these things are basically toys. And they're gonna cost money