Congratulations on your FIRE!
I'm not a complete expert on the ACA, but I did spend the night at a Holiday Inn last night. And I switched to the ACA mid-year in 2016 when I FIREd so I can describe how it worked for me and how I think it will work for you.
Assuming you worked the first part of this year and plan to leave your job and its income sometime in the middle of 2018 and you want to switch to ACA at that point:
You will go to your state exchange and apply for coverage to start in the middle of the year. Usually most employer coverage will go through the end of the calendar month of your last day, so for example if you quit today, April 10, your employer coverage would normally run through April 30th and you'd want ACA coverage starting May 1st. (There are some games you can play with COBRA if you want to skip May and start ACA on June 1st if you want to.)
So you'll apply for coverage starting May 1st. Since this is in the middle of the year, you'll have to have a qualifying event, which is the loss of your employer coverage.
If you want to get subsidies, you'll have to provide an income estimate for 2018. This is whatever you think the income your 2018 tax return will look like. It, in theory, should include any income you have received plus any income you expect to receive or generate via Roth IRA conversions. Note that most CSRs at the exchange are not familiar with early retire types and so you may have to explain your plan to them a few times, but eventually they will get it.
Your subsidies will be based on your income estimate and will be paid directly to the insurance company.
Come tax time for 2018 (about a year from now), you'll figure out the subsidy you received vs. the subsidy you should have received, and you'll true things up. This is done via Form 8962. If you got less of a subsidy than you should have, you'll receive an additional credit on 1040 line 69. If you got more of a subsidy than you should have, you'll have to repay some or all of it on 1040 line 46. Note that there are income-based repayment limitations, so you may not have to repay all of the difference.
And then in the fall of 2018, you'll estimate your 2019 income for your 2019 subsidy, and the process repeats itself. You'll be able to estimate a lower income. They may ask for an explanation, and all you have to say is that you don't work any more and so your income is lower.
As far as your question about RV'ers, I'm not as certain on that. My understanding is that most ACA policies are as you suspect: There is decent coverage if you get health care locally, and they'll cover emergency care if you're out of area. You can either call and get a preauthorization for the coverage, or if it's a super-duper emergency then you can go to the ER and argue with the insurance company after the fact if they don't approve the care for some reason. There is also special traveling insurance for RVers that I think I've seen mentioned.
@arebelspy has RVed around and so he might have a comment.