If you use niche.com you can pretty easily find median home values, median incomes, an some data pretty easily.

The mountain house has taken almost 2 years of breaking even and using some of the income on improvements until we have been able to cash flow.

I once bought a house in Essington (Tinicum Township) - 5 minutes from Philadelphia Airport and I learn that the small Tinicum Township had a lot of income from the airport economy: hotels, crashpads, parkings etc

They track management income, expenses, vendor payments, etc.

Given your experience with RV parks and MHPs, you already have a strong foundation for managing mixed-income streams, which will be valuable here. 

You can have multiple conventional mortgages and have a great credit score as long as you don't pay late or get too close to the maximum debt to income ratio.

Screen them as you normally would - does their income substantiate their portion of the rent +/- a little for changes in what the housing authority will pay out?

This can indeed provide a steady income stream and considerable financial stability over time.

I couldn't understand why their credit wasn't better considering their income was what it was and their debts were not unreasonable.

Correct - Track all your income and expenses.The bookkeeping will determine whether you are profitable or not.it will also be good to compare from year to year your income / expenses to see what expenses you can potentially try to limit.I.E.