I don't have a SMI system, but have a 'box' system which you place on the floor and plug into the cigarette lighter to power. I would assume the power consumption is similar between the two systems, as mine also has a compressor to pump up.
I can drive all day with no problem drawing the toad battery down. Still starts at the end of the day (which I do at the end of every towing day).
'07 Winnebago Journey 34H - CAT C7, Koni's, MCU's, SS Bell Crank, Safe-T-Plus
'07 HHR Toad, SMI AFO, Blue OX