How old are you now? Trying to read between the lines, it sounds like you are close to retirement age because you talk about having 401k savings and SS and wanting to know how much cash savings (combined with rental income) would be necessary to bridge the gap between now and 62. But the title of your thread states that you want to retire at 40.
If your intention is to live off of your 401k and SS at retirement age (62, in most cases), then does that mean you currently have enough invested in your 401k already, such that you have calculated that your 401k will be sufficient at age 62 to complement SS and provide for all of your living expenses? Do you have a benefit estimate from SSA? Or do you intend to live off of your 401k savings from the ages of 40 to 62, as well? That requires very careful planning to avoid running afoul of tax laws… How much is in your 401k now? How about your other accounts?
How many houses do you have? You wrote that you wanted to sell your house, but you also wrote that you intended to keep the $1,100/month rental income. That suggests that you have at least two properties. Do you own your own house (or houses) free and clear? What is it (they) worth?
“… selling my home and most of my belongings and exchanging them for an RV and never working again.” If you are under 40 and intend to never work again, your “home and most of belongings” should be worth a ton of money… in excess of $500,000, probably.
“… try to live as cheaply as possible so that you can leave the majority of your money invested” If you intend to live off your portfolio and also have that portfolio last a lifetime, it takes a ton of money…
You’ve received excellent advice on rental income. It *can* work out well. It *often* does not, especially if you are an absentee landlord.
As far as I know (and I am not certain), I’m the only person on this forum I know of that actually did what you are proposing: retire at 40 and live off savings. I retired at 39. I am currently 41. I do not intend to work again. I can tell you that at various points in my working life, I considered buying rental properties, but I always decided against it for all the reasons that others have mentioned in this thread. I have never regretted that decision.
You asked about numbers. As others have said, that is entirely subjective. If you have a gas- or diesel-guzzler and you like to travel a lot, your budget could be double what it otherwise would be if you just stayed on BLM land and moved 25 miles every 2 weeks. For me, I was happy with a general budget of about $1,000/month while I was living nomadically, but I found that “one off” stuff happened all the time, so to feel safe, I’d probably want to raise that to $1,500. I would want at least $3,000 (preferably $5,000) in savings to cover unexpected repairs to the rig. So, say $23,000/year ($1,500 x 12 months + $5,000 emergency fund). There are members of this forum living on much less than that. I ran the numbers once and decided that it was possible to be happy on the road with $800/month, but I’d want more because I don’t like living that close to the edge if I can avoid it.
Your budget for your rig matters a lot, too. You can spend anything you want on a rig. There are members here who report having spent only $3,000 or so on their rigs and being very happy. My rig was about $50,000 but that’s because I had a nice truck and slide-in camper. In the year I was living nomadically, it never broke down or gave me any headaches. That’s what I wanted, and I was willing to pay for it. If I were to do it again, I might get a van in good shape for $10,000, and put another $10,000 or so into the build, for a total of $20,000. Knowing myself, that would probably be the lower limit of what I’d be happy with.
It’s impossible to say much more than that since I don’t know how much you currently have, what the situation is with your property/ies, how old you are, etc. I can say that the kind of savings we are talking about, regardless of the details of your situation, is in the mid-six figures, probably.
I believe that that’s an amount that lies firmly in the realm of fantasy for 99% of people to save over the course of a lifetime; to achieve that by age 40 is outside the realm of reality for probably 99.9% of people. I have done it myself, and so I know what it takes. A lot of stars have to align for you to save that kind of money that young: high income, low expenses, low or no debt, and no wife or kids. Change any of those factors (lower the income, up the expenses, add the wife and kids, add crushing debt, etc.), and the chances of making it happen drop precipitously unless you compensate in another area.