Correct Timing at Idle for 75 GMC Motorhome

[campingman]

got a 1975 22 ft motorhome small block 350 stock HEI 73k miles. and going to be setting the timing on it. wanting to know what you all think the best timing would be btdc at idle? Im thinking 10 or 12 but need a refresher on this. Ain't worked on a small block in years. need to set it for heavy duty use since its a motorhome and will be going up some steep hills. Thanks for your alls input.