Dave, I looked into Oklahoma and USDOT rules before I setup my rig.  What I remember (Without doing an internet search) is.

At 1001 lbs (including the tank. If tank weighs 50, then 951 lbs => 150 gal), placarding rules apply.

At 26001 lbs GVW you must have a CDL.

I determined at 100 gals, I didn't fall under these requirements.

Mitch