Sorry I can't help you in person.
If you're overwhelmed by this, please get professional help.
You describe a rusty mess, and a detour sign as partial belly pan, which does raise some warning flags.
The black tank is inside a galvanized box, and might have to be removed to be repaired properly. Not that it's that hard to do, but with all the moisture in Houston, and the age of the trailer, you will likely find some deterioration that goes beyond replacing a seal.
I have not been cutting holes to repair my trailers, but have always been able to remove entirely or loosen partially the section of belly pan in question to gain access.
What's needed for your fresh water intake?