IPurchased an inexpensive 3" memory foam from Walmart and cut to size using an electric knife - it fixed the uncomfortable bed issue.
Your Mansfield Traveler model 911 or 910 is a Sealand and parts for it should be pretty easy to find. Do a google to find all the parts you need as they are available at lots of places. Scamp may even have some/many of them. Not much has changed over the years on those.
A Traveler Spec Diagram can be found on this website
with the part numbers etc. If you google the web or this site you will also find owners manuals for it, perhaps Sealands website would be a good place to start looking.
I don't see how the fresh water tank could have emptied out and filled the toilet & holding tank without the water pump having been left on. If the water pump is shut off the only water that can enter the toilet is water that was already pumped into the trailers water lines.
In my experience it is normally to see perhaps a few cups of water having drained out of the water lines into the toilet bowl while traveling which is why I close the lid when traveling to avoid splash out into the floor into the shower pan. I also flush the toilet before closing the lid to help empty out the trailers water lines so there is little water left in the system to drip into the bowl.
There is no connections between the black tank and the grey water tank so the water is not coming form there, unless of course the PO made some major change to the trailer. Easy to tell though. If you need to dump the black tank separate from the grey water tank they are not connected.