SO, that pic is of the bottom of your camper? Plastic panels for insulation and that vent is where the water is getting out? That is not your water tank. The plastic is just a cover holding in insulation.
Likely you have a pipe leaking, not the tank. It could be an easy fix, it might be the end of you.
You will have to fix the leak, it might have already done a lot of water-damage. First, look in the cabinets for water pipes and feel around for wet. The last thing is to remove that underneath panel and look for the pipe leak. Start easy, work up to hard.