For some extra space you can use a 'slide out' tray. this would make the size of your table top increase by about a quarter or more. When not in use the extra can just be slid back under the table.
If you do not want the slide out version you can make a 'slide-down' version. You would need to cut another table top exactly like the one you have and run the legs of the main table thru the second top which would mount 'under' the first top. When you are parked and having a meal, you just drop the second table top to about 8 inches and use it as a second surface. I can try and better explain if I haven't been able to. I am terrible at drawing but I will make a sketch if it makes life any easier for you.
The picture here is the slide out and foldable table that came original with the VW Westfalia camper.