It is city by city within county by county, within state by state within country by country substitute provinces, etc for state.
What this means is there is no uniform code regarding Covid 19 exemptions for dealing with people who are currently "homeless" as regards living/sleeping in a vehicle.
I do know that in a number of cities they have now started allowing (for the time being) more than 72 hours length of time for parking in one location because of the stay at home order as many people are not leaving during the day to go to work or out on other errands. That there have also been some temporary suspensions regarding longer vehicles being allowed on the streets where before they were not permitted at all hours of the day. There has been an up scaling of efforts to get homeless persons off the streets and into shelters with cities opening additional shelters and increasing the spacing to have social distancing inside of the shelters. In some of those cities it is against the law to sleep in vehicles, in other cities it is not against the law. That is unlikely to change just for Covid 19 although they might not aggressively pursue finding violators. But I would not suggest doing it in any city with an ordinance that forbids sleeping in vehicles as patrols have been stepped up at night in all the business districts to help prevent burglaries.
I doubt if much has changed out in the small towns and countryside except to get more strict rather than loosing up regulations. Doing that to keep out refugees from the large towns who might bring Covid 19 with them. On the Washington Coast towns have even shut down the motels and hotels, vacation rentals of all kinds including RV parks. No visitors allowed, we don't want you, we are telling you to stay away (for now) attitude.
But as I recall I saw on another forum in I think it was Bozeman Montana that they were allowing RVs and people sleeping in them to park on the streets overnight. It had to do with the mass migration of Canadian citizens heading north and US citizens leaving Canada coming back home. Double check that for yourself though as it is late and I am going to sleep now.