HF Radio..

always take your power supply from both batteries and connect to a 24v to 12v reducer. Otherwise you will end up drawing out of one battery al the time and it will shorten its life.
As a truck mechanic I have seen this happen on a regular occurrence, back yard jobs where people have fitted two way radios etc and just connect to one battery and wonder why they get a flat battery down the track after it collapses.

Regards
Jason

HF radios pull too much power (25amps peak) for most voltage reducers plus the reducers are very inefficient.
As has been said a Redarc Charge Equaliser is what you want, it floats both batteries at the same potential regardless of power drain from either.
I've had one in a 24v vehicle for over 4 years now with zero probs, just replaced the batteries as one dropped acell after 4 years.
HF's with autotune antennas are affected by low voltage and poor connections to both power, ground and aerial.

I think your radio has a 100W RF output? both my Amateur rigs, which are 100W too, draw around 20 amps when transmitting at full power which equates to a load of around 240W. In order to drop 24V to 12V you will need a power supply, either switchmode or linear, I would not use a switchmode supply with HF radio because the electrical noise it generates may (probably will) cause problems with radio reception. otoh a 240W linear supply is big and expensive - a switchmode won't be cheap either!

Given that most of your use will be with the radio in receive mode where it probably _Affordable_Storage_Drawers.aspx just an amp or two I would either connect it across one battery (the one with it's negative terminal connected to chassis) or I would fit an additional battery as a dual battery system and use an appropriate charger to keep it topped up from the 24V. The latter solution is better but more expensive (then again if you can afford a Canter motorhome you can probably afford the extra $200) but the former will work OK but may reduce battery life a bit and may lead to flat batteries if you Tx a lot.

Mike Harding