It would work but the Brake Buddy is cumbersome enough after you stop and you now add another dimension of lugging the battery around.
If you're recharging the extra battery, why can't you just recharge the toad battery?
PS - I've run 4 days cross country with the Brake buddy and have not killed to toad battery.
Tom and Katharine
'07 Winnebago Tour 40TD, 400hp Cummins
RVing for 19 years & 150,000+ miles