Per the original poster's question, I'm making some assumptions here. First, the site is reasonable level front-to-back; second, we're only staying the one night; and third, we don't need to go anywhere else that night. With those assumptions, I'll leave the mpg hitched to the car. I usually pull the electrical plug, but I've forgotten a time or two, and nothing bad happened. Of course, we were hooked to shore power, so if anything, the car battery was being charged.
Back to the leveling, I've had sites where it was level front-to-back, but off side-to-side. A board or two takes care of that.
If it is late when we arrive, and we're going to leave right away in the morning, I'll just hook up the electricity and maybe water, and not even worry about the stabilizer jacks. Yes, the trailer may rock a bit, but we're headed to bed anyway, so there isn't much of an issue.