Building a BIG BBQ/Smoker

So we have this 500 gallon air tank at school.
I found it at Bloch steel a while ago, and grabbed it on impulse.
My bosses are freaking out about cleaning the campus for our 40th 
anniversary of the founding of the school.
The reunion and party are next Saturday.

If I don't figure out a plan for the air tank soon it will have to be 
chopped up for underwater burning practice.

Seems a waste.
I am fascinated with the idea of building a BIG smoker/BBQ trailer 
for sale or rent.
I don't think I could justify keeping one unless I was renting it out
for $.

I can buy the tank from the school for the price of the steel, around
$150

Since the discussion has come up about Smoker BBQs I was wondering if 
anybody else in the Seattle area  was interested in building such a
beast.

Kind of a profit sharing venture based on participation.

I have found dozens of design variations looking at various 
smoker/BBQ trailers on the web, but I would appreciate any input from 
others.

I already have enough steel grating for the grill surfaces.

I am thinking a propane/charcoal hybrid.
One of the challenges is to find/build the trailer.

I did a search on eBay motors for BBQ trailers and was amazed at the 
number and variety of designs out there.

Many choices to make.