Welcome to the the boards and I really appreciate you sharing your situation, needless to say so sorry for your unfortunate experience.
Your willingness to share has educated many folks, including myself, on how to deal with our holding tanks. We usually dry camp, so this is not an issue for us. But when we do hookup in the future, I'm leaving tank valves closed until draining is necessary or we are leaving for the next location.
Again, so sorry for your incident and hope/pray for the best outcome for y'all.