Figured out how to fix it. I hope it doesn’t screw anything else up.
Go into your maps.txt file. Find your vehicle – helps I suppose if you gave it a unique name.
Then look for lines below your vehicle’s name that have numbers other than 0 in the second-to-last and third-to-last places on the end, like this:
1 0 -1 300 0 0 0 1 2 0
That 1 2 (bolded above) seems to be the problem (in another instance it was 1 1). Changing those numbers to 0 0 on each line defining a tile in my vehicle cleared up the error. I fixed about 8 or 9 of these, and my vehicle works fine again – controls are accessible again.
So, to be clear, the line above should look like this after the appropriate change:
1 0 -1 300 0 0 0 0 0 0
Glad, too, because this is a heavily tricked-out RV with tons of stuff in 7 trunks… it would have been sooo tedious to tear it down and rebuild another.