Proper voltages from same source
The proper way would be to use... a buck converter.

10A at 12V is only 120W. Assuming 80% efficiency, you're looking at around 3A out of the 48V, which is pretty manageable. You will need to design it carefully, a bit of simulation and prototyping, as you're looking at dumping around 25W of heat. A good design could take that lower, but plan for worse and give yourself some decent head room. Simulate it, find out what gets hot (and by how much) and make sure you design cooling accordingly.

Depending on your set up, there may be benefits from having a couple of 48V to 12V in strategic locations in the vehicle. That way you can take advantage of the lower currents at 48V, so that your volt drop is lower and losses can be managed better. If you do have multiple 48V to 12V, be careful to keep the grounds either the same, or completely isolated (take care with communication if you isolate some of the power rails!). 

Don't forget the impact of driving from the 48V though. Your drive motor will draw a lot of current (10A or so at 500W, more during stall events or heavy loads), so that could mean a significant voltage drop on the battery pack at high loads. Also, motors can put a hell of a lot of noise on the 48V rails. So you'll need to make sure your DCDCs can cope with all of this noise and disruption. That should be manageable with suitable EMC design.