There are many old threads on the subject of additional heating.
IMO, the safe and cost-effective method to add a lot of BTUs still
would be to install a propane water heater outside of the cab
, with a heat exchanger loop to warm the heatant
tank, allowing the stock heating system to operate normally, but allowing use of either the battery pack or propane for heating as desired.
There are numerous off-the-shelf RV and portable water heaters available, such as the one below for ~$120.http://www.campingworld.com/shopping/it ... odel/57523
Add any size propane tank you like, a 12 volt recirculating pump, and a heat exchanger to the stock heatant system, and you're in business.
The hardest part might be fitting all that under the hood, but if you wanted, you could also use the spare tire space for the heater and propane tank, and run the water (or heatant) lines to the motor compartment.