1. Welcome to the forum.
2. Sorry that you're having problems.
3. Keystone wasn't a part of the arrangement you had with the dealer to find someone to tow your RV around for you as you have no tow vehicle. In fact, I'd bet Keystone doesn't even know you don't have a tow vehicle. Their response is that if you have a problem, take it to your dealer and Keystone will pay to have it fixed under warranty. That's what manufacturers do.
4. I don't understand how a high pressure propane supply hose can collapse. I'm thinking that it (they) didn't collapse, rather the safety cutoff in the hose activated when you had a sudden rush of propane as it simulated a large leak. The supply hoses have a safety device to stop propane flow in the event of a significant leak.
5. As for the supply duct not being installed, your dealer should have found (and repaired) that problem during his PDI before you ever accepted the RV. That, again, is not Keystone, but your dealer.
6. It sounds like your repair technician isn't real familiar with RV furnaces if he has been out 4 times and it still doesn't work. It might be time to simply contact the dealer and let them know that you want someone who knows what they are doing to come out and repair your furnace problems. Hopefully, the dealer (not Keystone) will remember that you don't have a tow vehicle and will respond accordingly.
I don't think you can blame the manufacturer for this problem. Every RV made uses essentially the same appliances, whether it's a Keystone, Winnebago, a Forest River, or a Jayco, they all have the same brand furnace. Keystone will help you, but they didn't build the furnace. And, truthfully, the dealer should have fixed it the first (or at least the second) trip to your RV.
Good luck and here's hoping you get your problems resolved.....
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS