It would seem the desired thing would be a splitter cord that takes the TT-30 to both a TT-30 and a 5-15, so the trailer and car can both plug in, and you would just plan (hope?) to keep their current draws low, so you don't go above the limit. But I doubt that exists, since it seems like a bad idea electrically.
I don't really know travel trailers much. Can they use a 5-15 cord instead of TT-30 to run some basic low-current things like lights? If so, then at least the the couple of links to splitter cords people linked above would be a decent solution. Or, if you don't mind switching cords a little, treat the camper like a fancy unpowered tent overnight and let the car have the whole TT-30 while you sleep.