deniseQ
I like what toedtoes says and will add a little to it... don't buy an RV.
Not until you are sure you're going to use it and that you have the funds to spend on it. Buying used you are normally going to find you have repair or fix something... that can be expensive. Mine is only 8 years old and with less than 13,000 miles, I just replaced an AC and got the generator fixed and was over $800. A trip to a campground is going to average $60 a night and you are going to get about 8 MPG in a Motorhome so about $0.50 a mile. Also do you have a car that you can use as a TOAD or do you have to buy one of those also? or have you current one changed so you can tow it...about another $1800.
You said... "We are retiring next year and plan to go cross country and also use it for extended ( 2-3 week) trips." so not a steady thing. When you are not using in 2 or 3 times a year you have to park it or store it someplace is that going to add to the cost? Can you stay at motels cheaper in the long run. IF you can find a good one used for $50K and only use it over the next 10 years that is $5K+ yearly that you are spending... is it worth it? Can you use it more often fro other things than camping... Rent one first for a week and see how it goes before buying one.