Hi Pete,
When we were shopping recently, we were considering a Pleasureway Plateau, we inquired about having another (second) battery added as an option. According to the dealership (Poulsbo RV in the Greater Seattle area w/5 locations), it was definitely NOT an option from PW, but it could be done with modification. After they checked it out more thorally, they told us that it could be done with modification, but it would be a major modification, with big cost. We eventually bought a Great West Legend SE instead, and it has two fairly hefty batteries for the coach totalling 400 Amp-hours. The larger EX model has 4 batteries totalling 500 Amp Hours, I've been told. This is what I've been told--I haven't had any cause to actually take the floorboard covers off and look at the batteries myself. We've had no problems with the batteries handling everything we wanted, except for once when we tried to run the microwave through the inverter and it buzzed an alarm at us and we immediately turned it off. But according to what we've been told it should even run the Microwave with the inverter if the battery is fully charged.
So, there are ways to have multiple batteries in a Sprinter class B, so I presume that it is something in the PW electrical arcetecture that makes it so tough to add a battery.
Again, this is all stuff we've been told, not anything we've actually done, so please take this information with a grain of salt.
............Rocky