[fred cook]

Are you wanting a clear span or would you be willing to build in a mid span support? As an alternative to putting something in the water, you could brace from each side headwall up to the center of the span so that some of the load would be transferred (via force vectors) to the ground but leaving the stream free and clear. Also, I would tie the side beams together with steel to help stabilize the assembly. Your truck/trailer assembly will weigh probably around 15,000 lbs but the weight will be distributed thru the two truck axles and the trailer axles. Figure on about 10,000 lbs max point load (trailer). Considering the cost of your camping rig, it might be worth the price of getting someone to do the math and tell you what beam sizes you will need. Good luck with your project.