hi todd,
i'm suprised nobody responded to this yet but i'll put in my.02 worth just for posterity just in case you havnt figured this out yet.

this is a common issue not with just tennants but other brands as well like advance and boss. It even happens on lifttrucks that use this same type of tank venting system through the cap.
If there is a mesh screen just inside the cap area what the manual usually says to do when filling is to have all cylinders retracted and fill till you just see the fluid at the bottom of the screen. They say this is the correct level to fill to but they don't distinguish wether to do it hot or cold. Now most techs will fill it cold and of course when the fluid gets hot it expands and then starts blowing it out of the vent in the cap. What i've done is i run the unit till it gets hot and then put fluid in the tank till it just shows at the bottom of the screen. This does help but doesnt eliminate it completely. The fluid will slosh around in the tank and any residual fluid that gets into the cap will eventually start dripping out. I really never did like this type of design but it is what it is. You can always keep the fluid level just under the screen , this may help even more.

just a couple thoughts to think about for you and anyone else that may have this issue.