You can have tanks inside the vehicle if you have them in a dedicated propane locker, a totally sealed compartment with the exception of venting at the bottom (propane is heavier than air). Do a search on "propane locker" to see how they're done.
The lines run for propane are not the biggest problem. The bigger problem is leaks at the connections. Therefore, the correct way to do propane is to put the tank in a vented locker, hook up a solenoid valve to open and close the flow, and then have the propane distribution (if any) take place in the locker so that a separate line with no intermediate connection runs to each appliance.
A lot of people don't want to use propane in their RV because of the perceived danger of explosion or fire. But if the installation is done to the proper standards, propane can be a safe and efficient fuel.
2007/2012 custom Jeep Rubicon XV-JP motorhome
2003/2014 custom Sprinter 2500 mid/tall motorhome
2002 Toyota Sequoia Limited 4WD
2006 Honda PS250 Big Ruckus Expedition Scooter