I've never done on on a GI trailer but have done or been involved in several stationary and trailer BBQ pit builds. I'm currently looking for a 3/4 ton chassis to make a MV show and rally pit.
A lot of folks have made a lot of both kind of pits and both can make great product with little heartburn.
I'm a fan of using a separate wood fire box with a used propane tank for the smoke chamber. then I'd add 2 propane burners-one to light the wood and a second, with a wood pan for smoke, in the smoke chamber for times when I don't want to use wood or as a boost heater for really cold, wet or windy days.
A 300 pound upright tank is usually about 5' long and can be set up to feed 60 or so folks with ease. A 300 gallon horizontal tankt is usually a little over 90" and can, depending on what you are BBQing can feed 200 or more.
The propane tank is longer and narrower making it easier to reach all the product in the smoke chamber. It is stiff enough to not need much bracing and will make temperature regulation much easier. It won't warp and is easier to get tight fitting doors that stay tight. When you're doing most of your cooking at 225-275 degrees, that's a biggie
With a propane tank the door(s) usuallyare higher in relation to the tank's height so you can put from 2-4 racks in it and use 2 racks for big items like hams or shoulder clods or use 3-4 racks for thinner stuff like rib and brisket.
If you use upright rib racks you can really pack 'em in there.
I've done very little BBQ in a 275 gallon oil tank, so take my thoughts on them as such.
One big advantage to the oil tank is that in laying flat it's able to accept a whole butterflied hog easier. The depth, front to back, means you'll be leaning over the cooker more and I don't know if it can be safely done on a 2-1/2 ton chassis because of height.
Maybe you'd need a work platform? Perhaps a 300 pound tank or 275 gallon oil tank crossways on the rear with the rest of the chassis being used for wood, water, cold storage, fish fry burners, yadda, yadda yadda?
I think the biggest challenge will be the height and width of the trailer. As bulky as a 2-1/2 ton trailer is you'll be a long way from most anything on it unless you really take care in the layout.
I'd consider coming up with your best plan and then mocking it up with cardboard boxes and maybe a 55 gallon drum and then pantomime a cookout.
Heck, the neighbors probably already think you're nuts so they won't mind. See how it all flows from prep to cooking to serving and clean up before actually building.
If you web crawl for a while you'll see plenty of ideas. Here's on site that has some well thought out designs.
Barbecue Cooker, barbecue grill, and barbecue trailer from Cookers and Grills featuring unique and efficient designs of barbecue cookers, barbecue trailers, and barbecue grills
I have a CD that walks through the design and construction of a propane tank pit. If you want a copy, PM me and I'll see if I can find it. I'd have to get some blank CDs on a trip to town.
Lance
EDIT: gohot's got exactly what I was thinking.