So this will be my first BM (my schedule willing). But I don't know if I can get there on opening day. If I get there on a Wednesday, can I just park somewhere and sleep in my car? This isn't my plan but worst case scenario.
In theory, yes. Park in any unclaimed camping area.
And many who arrive in the city late at night do that *their first night* to avoid pitching a tent in the dark. But four or five nights in a row, when the temperatures can range from 50-110 over the course of the week (it will be inhabitable by 11am each day as the heat hits).
That will not be easy, my friend. I recommend a tent of some type. Unless your car is an RV, has a trailer attached, or is a pickup with an air mattress and an awning.
A minivan is doable also and putting shade over it will help with the heat. The other big problem the getting all the F@#$%ng dust out of your car. By Wednesday however you'll be parking in the stick, and it's a LONG ASS DRIVE.
One year I encountered a Japanese journalist who was sleeping in my buddies trunk. The crazy part was when they took me over to meet him, it was like 2 or 3 in the afternoon and the guy was just chilling in the trunk of a car. It was at least 100 degrees out. He didn't seem worse for the wear either. I guess it is really a matter of perspective.