Welcome mate. We need more info from you like how many people, what you like to do/see and the type of RV questions is not clear. There is a lot of info in the two boxes at the top of the page.
For two people moving often I would suggest something like
this. Other than maybe putting new tires on it, you should be able to use it for a year without any other major expenses. Small enough to get around in yet big enough for when you cannot get outside on a rainy day. Should get 10 MPG and the tanks are large enough so you can dry camp (no hookups/saving money) for 5-6 days if you are careful on water usage.
Your budget depends on how frugal you are and what all you want to see. If you plan well and only make one circle around the country, not back and forth, your fuel bill for 16,000 miles at $0.40/mile= $6400. CG fees can run from free to $100+/day in the winter (southern US) but I can average less than $20/day so $7,000 for one year. Makes no difference if you eat here or down under so I do not figure the cost of food. Admission fees can be $100+/day/person (Disney) or free so that's something you'll have to figure.
