In the Great Recession we converted these units to LTR in the school year and STR only in the summer.  

This is a microeconomic question:  Are people going to "postpone" or "cancel" their discretionary income that they would have spent on an STR on vacation?

That’s why I never recommend to invest for equity (whether an LTR or an STR), only for cash flow.

Having both vacant on the same weekend I thought would never happen again (in the Great Recession they were converted to LTR in the school year and STR only in the summer due to vacancy issues with the STRs). 

They might not have the budget for a STR on par with our normal rates, but perhaps at a significantly reduced fee, we can at least get some incoming cash flow to go towards the mortgages, and they get a clean, private space that they wouldn't have otherwise been able to afford. 

My family owns a STR on the beach in Manasquan.

I would buy a STR on a ship!

If so, you could buy a multifamily and keep one unit for yourselves and do STR on the other unit(s).

I am seriously exploring a BRRRR STR on a SFR in San Francisco which started out as a LTR.

I've tried doing STR on a couple apartments but haven't really figured out the right formula yet (and it's not my focus), so my experience with sites like Airbnb is super limited.