The Regional Championship consists of 10 races with the specifications:
A Championship of Class *D to S, R3, R2, R1* from *Country/Continent*
There's not a trick in winning these ones. As there's a restriction in class, you can't upgrade your cars that much.
If you for instance do the 'Asian cars from Class D' races, you should push the D Class to the limit, to 400 if possible. This way your car is still D but has a huge advantage over the other cars.