@Carl A.
I have the same situation. National tenant and other corporate tenants. We sent out letter stating what we would offer and also an addendum that they would sign us as beneficiary of any of there PPP that is related to rent.
All of these tenants are not helping themselves. We will asking for larger security deposits in the future. The smaller tenants mom and pop have paid or at least have attempted to pay something.
In summary you should give THEM 3 options. Have them choose one and if not they are in default. Not legal advice but if you are offering other ways to pay ie 1/3 rent per month or 1/2 something along those lines, when and if you go to court you can show that in good faith you attempted to assist. 90 days not to pay rent is a default on lease terms. Purchasing retail and having “national tenants” you are burned the worst with them 2008 and now. Mom and pop stores and business much better as tenants then the national tenants. My opinion and personal experience