If you didn't want to have to use tie downs, you could construct a mount similar to you see for bicycles in the back of vans/trucks. Take a small sheet of plywood, about the length of your truck bed side-to-side and about a foot wide. Cut a piece of 4x6 to fit between the fork tubes and drill a hole for the axle. Lag bolt this to the ply wood and now you can simply run the axle back through the front end and wood block and you bike is secured. Now no need to compress the forks when securing the bike. With a weldor and a bit of materials, you could make a more elegant solution, but the wood is easy to work with and most people with bikes also have the tools necessary for that type of work. Just and idea for you. Something similar could bee done for the rear if doing the back of the RV thought. You would need to secure it to the floor somehow though.