Like they said, usually if you have an enclosed underbelly, you also have ducting and insulation under it. That enclosure helps keep the insulation up and keeps your trailer warmer in colder weather. The tradeoff is that you have less access to the underside of your trailer.
You can take off the zip screws or cut a hole in the panel to get access if you need it.
You are likely right about the bar being some sort of strapping to hold the tank on. I've heard very infrequent horror stories about improperly supported tanks falling out when full of water... but I've never seen it in person. Regardless, I try not to drive too far or too fast if I have water in any of my tanks.
