Have you looked into getting a (used) scooter?
My wife is abour 350# and has one(3 wheel). And I've been to event were gals your size have had 4 wheel ones. Used ones can be had for about 1/2 price if that is an issue.
We had a lift installed in our van (removed 3 of the removable seats), others have had a ramp of various sorts installed or placed in their vehicles.
Just a thought