Sorry for your troubles, dbutler, and welcome. Shame that this is your first post but glad you're here now.
It sounds like Sunseeker and Willhound are on the case.
City water filling the fresh tank sounds like classic stuck check valve in the pump (and if you haven't used the water system in a while, it's pretty much guaranteed to be a problem). Switch back and forth btw city water and pump, opening a faucet between switches will "exercise" the check valve by putting pressure on it in opposite directions and is a solid way to try to unstick it.
Water coming out of the fresh tank dump valve is simple the dump valve being open. Some people store with the valve opened, some closed. If you were running on city water with a check valve properly working, the fresh tank wouldn't have had anything filling it. Closing the dump valve dealt with this "leak," but it didn't address the faulty check valve.
Capping the overflow vent was a mistake (not sure how you managed this, I've not seen an overflow valve that could be capped). As Sunseeker mentioned, this vent also allows air to escape when the fresh tank is being filled/displaced by the incoming water. Capping it kept air from being able to be displaced, and the tank (which is large and flat, like a book) distorted (attempting to become "round" like a balloon). This is actually a real problem, in that overflow vents can become blocked or inneffectual for all manners of reasons, from blockages to being kinked (to owners unknowingly capping them) and the tank expands upwards and down.
Typically, if the tank is lightly suspended (and they are) and limited against the bottom of the floor of the coach, then the tank is forced to bow downward till it distorts so much that it is no longer held by the supports (when bowing - and gaining vertical height - the tank loses horizontal width).
If the tank is WELL suspended (and reinforced, which some new trailers are being, or if reinforced by an owner), the tank will attempt to expand *upward* and, as a result, can buckle (and break) flooring above it. Even very modest pressure into a closed tank will fill eventually it and can create a lot of pressure (have you seen the
exhaust run jack bags, that inflate using the exhaust of your car?)
So:
- Bad check valve = fresh tank filling from city water.
- Closed tank dump valve AND overflow vent = tank effectively becomes a "balloon"
- City water pressure pumped up the tank like a balloon = tank deformed and it (plus its new water weight) popped it out of position and fell, striking the ground (and tearing out some lines along the way).
- Crack around the bottom drain fitting = (likely) from the drain fitting hitting the ground with the added water weight.
If you never EVER use your fresh tank, I supposed you can delete it and cap the fills to it, but I'd suggest you probably want the tank for the random chance to overnight (or boondock), plus resale value down the road.
My understanding is that fresh tanks are made from HDPE material that do NOT like repairs (According to Elkhart Plastics (Elkhart, Indiana), which supplies the RV industry with holding tanks, RV tanks are most likely High Density Polyethylene/HDPE), so you are likely on the hook for a new/replacement tank with fresh hardware.
During construction, tanks are laid into the frame from above and then the floor is laid over the top, so you likely have to have one of the angle iron supports CUT so that you can put a new tank back up into there from below. Then it's a matter of inspecting and re-installing anything the tank tore out on its downward fall. You might also take a look at the floor above the tank, and look to see if its ballooning upward did any subtle damage (cabinet frames cracked, floor out of position, etc).
Likely a big project, but not a complicated one.
Again, sorry for your woes. Just my .02, hope this helps.