I'm not a fan of mobile home parts for anything. The secret to mobile homes is that they are made light and they are made quickly. They are not designed for allot of travel, and they have a terrible history of falling apart while just sitting still with cement blocks all over the place to support them. I wouldn't even guess what thee frame could support at the center when supported on either end, but I bet it's not going to be very much. Add the lumber to create a deck on the bridge and you're probably near the limits of what the frame can support.
Ideally, a 54 foot flatbed trailer would be perfect if you could find one for a reasonable price. Odds are that it would be a rare find, but you never know.
There are some charts and of course, engineers out there that can tell you the size of a pair of I beams that will support the weight of a UTV at midspan. Once you know the size you need for the beams, the rest should be easy.
Good luck,
Eddie
