Hello Smiley,
I'm sorry to read of your collision, and the problems you are having. I hope you get better quickly.
I totally agree with you that if you property is damaged by someone else then it would be nice to have it put back the way it was as if nothing had happened. Unfortunately Insurance doesn't seem to work that way, and the ugliness of economics rears its head.
To be fair, as a caravan or car gets older it becomes more difficult to repair, sometimes its the difficulty and cost of finding the correct parts, or it can be the repair will be 'stronger' than what it is attached to which can then cause further failures quite quickly.
As this is an insurance claim, the insurer will be liable for the quality and durability any repairs (after all they are paying for them) and with a longer view, and if they can see possible future issues with the vehicle which may or may not be part of their liability they would prefer to circumvent the problem by declaring the item uneconomic to repair.
This doesn't means it can't be repaired, it simply means the insurance are not prepared to repair the item for you. Usually any offer to repair or replace made by an insurance company has an equivalent cash value and you can negotiate to receive the cash, and then you can use it to arrange your own repairs.
Even though the other party may be totally responsible for the losses, you are still making a claim, and you are duty bound to inform your insurers as it is a material fact and they ask you to declare any claims you make. Depending on the type of cover you have, your own insurance company may take on the claim on your behalf and negotiate with the responsible party.
The most important thing is don't be bullied by the insurance company. If you think the offer is not good enough you can appeal the decision, though it very unusual for an offer for this sort of claim to have any large uplift.
Hopefully Raywood may add a few thoughts and insights.
Good luck