Don't worry about sounding like an idiot asking all of those questions.

I've recently gone through a few things where it was my first time of doing it, including putting the RV up for consignment, and I let them know right up front that I've never done it before so please excuse me in case I do need to ask a lot of questions or I don't quite understand.
You should clean the RV up as much as possible so that the dealer is interested in even taking a look at it. That being said, they will detail it before it goes on their lot too.
I will let you know of my experience and see if that helps. We did clean it up as much as possible as though we were selling it ourselves. The dealer did some repairs on it and charged us for those repairs before we could consign it. We were instructed to keep it registered and insured until it was sold. No need to fill the water tank. You could probably keep a little bit of fuel in there and propane, but not much. Don't worry about filling those.
Many dealerships will work with you on a cash value that they will give you. So, say you would be happy if you could get $20K out of your rig, then you sign a paper with them that says that is what they will pay you. Then, they get to charge whatever they want on top of that to sell the RV. This is the style that the RV dealership we used does.
Some will list the price of the RV at 45K, and then take a certain percentage of whatever it sells for.
You would want to call around and ask some folks, but don't be shy. Just tell them your situation. Tell them your RVing needs have changed over the years and you have an RV that you aren't using anymore and that you want to know the benefits of consigning an RV with that particular dealership.
I spoke to a guy named Gunter at Giant RV in California and he was SUPER friendly and easy to chat with. I would have totally taken my RV to them if I lived closer. Call them and see if you can chat with Gunter. If he can't consign it for you, I bet he would be able to lead you in the right direction.

We signed a 90 day agreement. Once the 90 days are up, we could get our RV back again or sign another 90 day agreement.