I am making a 3d cake for my bf's birthday in a month i am doing a smaller version of it first to see what all has to be done. I am going to make the trailer out of choco cake and what recipie should I use to dirty ice it? also what kind of filling can i use between the cakes? The truck part is going to be made out of rice krispies, can u use the same butter cream to ice it as i did the trailer?

......what recipie should I use to dirty ice it? ,,,,,,,,,,,
whatever icing you ha ve always used up till now

......what kind of filling can i use between the cakes?........
Filling can be purchased at cake supply stores in what is called a sleeve. It does not need refrigeration once in the cake. (If you have any leftover still in the sleeve, it is best to refrigerate it.)

..........The truck part is going to be made out of rice krispies, can u use the same butter cream to ice it as i did the trailer?............
Why not? You might need to give it a thicker coating to keep it smooth as RKT does not produce a nice, smooth finish like a cake does.

Petermobile, why not melt some cho. wafers and brush the melted cho. over the rice krispie tractor and let it set up and feel it and if you still need a little more to make it smooth add a little more. Then Ice your tractor with the buttercream or maybe you want it a little rough so the buttercream will stick to it and not move around on you as you are applying it. I know that fondant would show the bumpiness of the rice krispie treats, but then that might help the buttercream have something to attach to. I would suggest making some of your treats or buying some and try both ways with the icing before even making your tractor. Then you will know which is going to work better. I am sure that I could say that we would all love to see your tractor trailer cake when you are done with it. Best wishes.