For an accurate time on battery life you would need to know the battery : Amp Hour ; rating . My 2cw, you should have 6 for boon docking.
Engine HP, is only 1/2 the towing equation , you need to know the trans rating too. Some manufacturers are installing a minimum rated trans , say 30,000 GCWR and exceeding that may result in early trans failure.
Again my 2cw. Look for a coach with 350+ HP and a tow rating of 10,000 lbs., Allison 3000 series or better. You won't regret the extra $ spent , easier to drive and fewer worries about issues due to possible overloading.