I used to park my 6v vehicles on hills (40 years ago), until I learned about incorrectly-sized battery cables AND straps, and until I learned about bad grounds/connections due to rust/corrosion.
My guesses regarding the cause of starting problems "unique/common to 6v":
75% of 6v starting problems - wrong straps/cables
20% of 6v starting problems - bad connections (there are 6-8 cable/strap connection points)
5% of 6v starting problems - bad/weak/old battery - one of my 6v Optima batteries has lasted over 14 years
I have no problems now starting my 6v vehicle, even after winter hibernation.
OF COURSE: if you need 12v devices that draw a lot of current, you would most likely want to completely convert 12v.
