For something simple look at the extra500/Systems/extra500-system-fuel.xml, line 67
This is just a simple interaction between the air temperature and the fuel temperature. The sun radiation is not accounted for as when you are flying its influence is can be neglected. The solar power is only of importance when you park your aircraft for a long time and I don't usually do that in FG...
This is for a carbon fibre wing, so if you are modelling a metal tank/wing, you will need to increase the energy transfer rate considerably.
I also assumed the area only on the bottom as the fuel tank is usually not completely full and air is a very good isolator.
So you need this formula:
https://www.engineeringtoolbox.com/cond ... d_428.htmlWith dT the temp difference between the air and the fuel.
after you have calculated the energy transfer you can calculate the temperature change of the fuel with the specific heat capacity of your fuel (AVGAS of JET-A1).
Cheers,
Eric