I first wonder why your roof has to come from Lance. I used to own a 1990 Lance M900 with an aluminum roof that was hit by hail. In my case they just totaled the camper as the aluminum sides also had damage.
I considered fixing it but decided to sell and look for a Bigfoot. It appeared to be a one piece sheet of aluminum held on by screw at the top of the walls and by the items on the roof. If you can source a sheet in that thickness I have a hard time believing it won't work.
Have you to talked to other RV repair places? There are some that either specialize in roofs or at least do a lot of them. Maybe they can quote by picture. If they are not too many hours away that may be an option. A metal fabrication shop could probably build on as well. With the old one removed and on the ground you have the perfect template from the old one.
I would suggest calling Lance directly for two reasons. First, verify with them that the part is obsolete and there is no dealer stock. I've had many a dealer tell me a part is obsolete when there was still stock in warehouses. Maybe Lance would would share with you who the manufacturer was for the roof panel. Second, they may be able and willing to give you detailed structural drawings of the camper. They were able to supply a few years newer than my unit to me and it was very helpful for repairs. You could also ask Lance if the roof is structural and ask them to write a letter stating so. Also ask for a letter stating if a rubber roof is an acceptable retrofit and if any alterations need to be made to accommodate it. If it is not acceptable that is ammo against the insurance company.
If the shop will write you a letter stating the roof is obsolete from Lance then you could try to get them to total the camper. You almost certainly have coverage that is supposed to put the camper back in the pre-loss condition. A rubber roof was not previously on it and I personally would consider a rubber roof a downgrade and ongoing maintenance item. If you bought your camper used at a good price or were considering upgrading this might be to your benefit for it to total. Of the flip side insurance may not pay enough to get you in a comparable unit. That number can be argued as well.
Hopefully that gives you some avenues to explore.
As others have said coating it may be a valid option too especially if it is small hail. With larger jagged hail and deeper dents I would have long term concerns with small cracks appearing in the divots after several years of heat cycling. Coating may prevent/stop/cure that too.
1988 Bigfoot C-11.5 on a 2002 F-350 DRW 4x4
