I know AirPrint only prints over WiFi so that's probably not an option.
AirePrint via WiFi is the only way to print. There are printers that include their own router. It called adhock printing.
Are there any good app/bluetooth printer combinations that anyone has worked with from a vehicle?
Apple doesn't supply a bluetooth print driver.
Mobile printing.
This blogger found one solution.
http://genuinely-original.blogspot.com/search/label/iPad%20Printing
Make Portable Printing Easier with Print n Share and the Brother RJ-4040
http://prmac.com/release-id-44494.htm
Use an airprinter with a power inverter
http://genuinely-original.blogspot.com/search/label/iPad%20Printing
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List of supported AirPrinters.
http://support.apple.com/kb/HT4356
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You might need a router with this solution.
Buy another box
"xPrintServer enables wireless printing for iOS devices such as iPads® and iPhones® using their native print menu. It eliminates the need to print through apps, install software, or email yourself documents for printing. And more importantly, you don't need to buy a new printer because xPrintServer works with more than 4,000 top brand USB and network printers. "
http://www.lantronix.com/it-management/xprintserver/xprintserver.html
You have to check whether this box supports your printer
http://www.lantronix.com/it-management/xprintserver/xprintserver-supported-print ers.php