Hmm...
Since noone mentioned it yet.
There's some DOT (Department of Transportation) regulations about transporting pressurized tanks in the US.
You can have up to 1,000 pounds of tanks (gross weight of tanks/valves/contents) in a vehical with no problem as long as it's not for any 'professional' purpose.
If you have 1001 pounds or more, you must put hazard placards on the vehical. NOTE: If you have hazard placards, the driver must have a comercial licence for hazardous materials transport (so keep the total under 1000 pounds unless you've got one).
It's generaly not enforced BUT if you got in an accident with a bunch of tanks on board over the limit you might be in for some trouble.
Also, if you are transporting the tanks for a 'professional' purpose, you need to have a bill of lading in the vehical (basicaly, just a list of the tanks and their contents). Again, noones gonna check if you have one UNLESS something bad has happened. Then if you don't have one, you might have a problem. EG: if you are traveling to/from a divesite to teach or DM a scuba course, you should have the paperwork even if you are only carying a single AL80 for your own use.
After all that, I have never heard of anyone getting in trouble for breaking the above regulations in relation to SCUBA diving activities, but it's something to be aware of if you're gonna build a trailer that could some day have a whole load of tanks in it...
Last, you should research the regulations I've mentioned above for yourself as I'm going from memory here.
