I thought it was a battery drainer (as compared to my Nokia 808 Pureview running Nokia Maps) on my 1020 but I then realised it was a problem of charger and cable.
Basically:
a) if you use a data cable (or charge and data one), intensity will be limited to 0.5A
b) if you use a charge only cable, then no limitation; if your charger is 1.0A, then you get the 1.0A on the phone
Now when you look at your Nokia wall charger, it uses a data cable but the charger has its own data pins shortcircuited. In other words, whichever cable you use, it will behave as a charge only USB cable.
So the alternative is: you look for a charge only USB cable, or you look for a charger which "blocks" the data pins.
I didn't want an extra cable so I went for the Nokia DC-20. I use it with the USB cable shipped with the 1020 and it works great: on a 4h trip, 4G on, BT on, WLAN on, MixRadio on, Drive+ all the time = 100% battery during the whole trip.
Another solution is to lock the screen when you drive on straight road (like the next turn is in 70km). It will save a lot of battery and the vocal guidance on Drive+ is a lot more precise than the one we had on Symbian: you don't necessarily need to see the map to understand where to turn right or left. Drive+ resumes very fast when unlocked, anyway.