Biggest problem at your age will be health insurance. You're still 'youngsters' but the body has this way of surprising you when you get around 60 and most times it isn't trivial, so make sure that you're covered.
Generally we find that people spend what ever they have to spend. If you are planning on less than $2500 a month then you are going to limit yourself. Not saying it can't be done, but it won't be as comfortable as it would be if you have more income from retirement plans/savings.
A lot of your expenses will be the same. So the first thing is to take a good look at what you spend your money on now, identify those things that will stay the same - like food, eating out, recreation, then look at the things that will change - mortgage goes away maybe replaced by RV Loan, utilities replaced by campground fees, more fuel, less on clothes (no more work clothes), and see what range your total is. Yes, a lot of people put their actual expenses on line, but we've put together some things to think about on our blog:
Budget Considerations
Take a look and see what might apply to you. Obviously we all have things that are unique, so unless you are really into wine like we are, having a separate budget line for wine won't make sense for you.
Barb