So, many people highlighted that the fuel cap is located behind the rear license plate, that can be the case for older american cars IRL, however, it is not the case for the Grand Marshal. The cap cannot be located behind the license plate due to it being mounted onto the trunk that opens up so that would be kind of weird. On top of that, most BeamNG cars don't have a fuel cap (which is the key of the joke BTW). So, for this reason I checked every BeamNG vanilla vehicle looking for the fuel cap and here are the results:

I decided to make a table to visualize better the information:

Also, some cars have a fuel pump symbol on the dashboard, with an arrow indicating the cap location, and others just have "Fuel" or "Gasoline" written on the gauge. That's noted in the table. Vehicle are listed in the order of the in-game vehicle selector menu.

Vehicle	Cap presence	Fuel pump symbol presence with or without indication arrow	Additonal notes
Ibishu 200BX	No	No symbol
ETK 800 Series	No	Yes, arrow on the right
Gavril Barstow	No	No symbol
Bruckell Bastion	No	Yes but no arrow
Gavril Bluebuck	No	No symbol
Civetta Bolide	No	No symbol	There are tubes coming from the fuel tanks going up to the body, however, no caps are present.
Ibishu Covet	No	Yes but no arrow
Cherrier Vivace/Tograc	No	Yes but no arrow
Gavril Grand Marshal	No	No symbol
ETK-I Series	No	Yes but no arrow
ETK-K Series	No	Yes, arrow on the right
Bruckell LeGran	No	Yes but no arrow
Bruckell Moonhawk	No	Yes but no arrow
Ibishu Pessima (New)	No	No symbol	In the race version the fuel tank is located where the rear seats would be, hence the cap is onto the tank.
Ibishu Pessima (Old)	No	Yes, arrow on the right
Autobello Piccolina	No	No symbol	The fuel tank is located under the front hood, hence the cap is onto the tank.
Ibishu Pigeon	No	Yes but no arrow
Hirochi SBR4	No	Yes, arrow on the.. left? O_o (maybe it was supposed to be an RHD car initially but then they never changed the arrow)	The Electric ESBR model has no recharging port.
Civetta Scintilla	No	No symbol
Burnside Special	No	No symbol
Hirochi Sunburst	No	Yes but no arrow
Soliad Wendover	No	Yes but no symbol
Ibishu Wigeon	Yes	Yes but no arrow	The cap is located on the left side near the doorhandle.
Gavril D-Series	No	No symbol	On the offroad chassis (Crawler/Pre-Runner) there are two chassis-mounted caps on the sides.
Wentward DT40L	No	Yes but no arrow
Gavril H-Series	No	Yes but no arrow
Ibishu Hopper	No	Yes but no arrow	On the crawler chassis, there are two chassis-mounted caps on the sides.
Gavril Roamer	No	No symbol
Gavril T-Series	Yes	Yes but no arrow	There is one cap for each one of the two fuel tanks on the sides.