for the most part of our actual travel time, we put our tanks in the bed of the pickup.. I made a "holder" to keep them from rolling by tieing PVC pipes together with ropes spaced apart just enough for a tank to sit between each pole...and each pole is inside of a cut to length pool noodle to pad it...works great...we use the same "flexi rack" on the floor of our boat to keep them snug and not roll around.
when we have, on occassion, wanted the tanks to ride or store safely in the TT, I simply place that same noodle covered pvc rack on the floor right inside the side door and slide the tanks in...easy on my back, and they are placed nicely near the load bearing axles...there's a solid built in counter on one side, and I secure the tanks with dive weights and dive belts laid across the top of the tank stack (never more than 2 high)..we've never had an issue either on boat, bed of truck, or floor of camper.
The most the wife and I ever take is a total of 6 tanks (3 each) but usually that's when we plan to dive Largo and we're just pulling our boat for that trip...not the camper...so again, bed of truck for the tanks.
when we do trips to mid Fla...ie Rainbow river, Ocala springs etc...that's a camping trip for us...so we usually just pull 4 tanks down for that...and I only put the tanks in the camper once we are someplace parked where I would worry about theft.