USA roadtrip - rental cars and mountain bikes

Proper security for a big old Enduro machine is a great whopping big knoife. No, wait that's Aussi. How do you say knoife in Kiwi? You've got big gnarly sticks and the baddest screw face in the land. That might be all that you need. Otherwise, try to fit it all in the rental car, and/or carry a big lock and/or scary big stick.

On second thought, perhaps quality rental insurance is the ticket, though that doesn't help much when it's time to ride and you have no ride.

Or you could just trust in your fellow human beings. But that's a crap shoot. But I tend to do it anyways and I've been good so far.

Out of sight, out of mind. Be sure to cover your bike with a blanket or sheet if possible, and bring a lock for inside the car just in case. When I road tripped through NZ last year, we stopped at a local thrift store to pick up a cheap used blanket to cover our rides.

What areas do you plan on visiting?

Are you sleeping in the car or in a hotel room? Put the bike in the hotel. Problem solved!

Or, make sure your rental car is large enough to keep the bike inside of. Cover it and lock the doors. Insure the thing if you're nervous about it - it's never a bad idea.

i'm not sure if this is a legit possibility but when you visit the big cities, if you know you're not riding for a day or more and will be away from your rental car doing tourist stuff, maybe a local bike shop would be willing to store your bike so it's just not sitting in the car. bring 'em beer.

You might want to consider the vastness that is the US and Canadia. When my cousins come here from Britain they think it's like a couple hours away between Texas and Oregon. In reality, if you're driving, it's days away. So, take in your map projection and be realistic.
To get back to the question though....locks. Lock it up and like Turman says keep it out of sight. A black blanket is less alluring than a shiny bike. Ask your rental car company if their insurance covers theft from a vehicle or if they offer something to cover your spendy bike. I've had to bump up homeowners insurance here just to ensure my bikes and cameras will get replaced if jacked.