Subrogation means, in a nut shell, your company "loans" you the money for you to get your car repaired or the value of your car, based on your policy amount. In turn, you give your right to recover the damage over the your company. This meets your obligation to the "loan". It's not a loan you have to pay back. They are obligated to pay you by the terms of YOUR policy. They have no legal right to any more money than they have "loaned" you. It's not that they WON'T help you...it's that they have no right to ask for more than they paid. They used to have you sign a "Loan Agreement" in order to show they had a right to the money owed you, but that has kind of fallen by the wayside in most cases. Bottom line is, they cannot "work" for you to get anything they haven't paid and they have no obligation to pay you more than your policy limit (agreed value).
As for the concern about approaching the $7000...and I'm going to have to read my policy on this one...IF the "agreed value" is $7000 and you and Hagerty have already agreed that IS the value of your car, and the salvage value of the car is $1000, then you would only get paid either $7000 and they take your car or $6000 and you keep the car. If the cost to repair is $6500 you'll be losing money by going thru them. I'm not sure if the $7000 is a "agreed limit" on the policy or an actual two way agreement stating that the two of you(you and Hagerty) have "pre-agreed" that IS the value of your car. I'm going to check the policy, because that could most definitely affect the amount I want to carry. I am currently carrying enough to cover only my investment...which may be my mistake.
Under liability laws the other (trucking) company or their insurance owes you the cost of repair up to the Actual Cash Value (ACV) less salvage value. Most states, and I don't know Washington, have a percentage of damages as compared to ACV at which point you have to get a Salvage Title and this establishes the basis of what is a "total loss". Some don't require a salvage title if the car is over a certain age. In this case, if you car is worth 8000 and salvage is worth 1000 they would pay up to 7000 to repair so you would be ahead to go thru them. As I have said before: Move slowly and ask questions.
As for the other company using your $7000 agreed value to limit their amount. 1) they have no way of know that unless you tell them 2) It's easily explained that you insured it for your investment in the car, not the ACV. Randall is right, they could try to use that as negotiating point, but it won't hold water.
I am not a lawyer and also not familiar with Washington state laws, so I'm not giving legal advise here, just trying to help you understand what your options are. That's my disclaimer. I retired after 35 years of handling claims, mainly in KY but some in other states. I'm somewhat amused at myself for even discussing this stuff, after being so happy to get away from it when I retired 3 years ago. I'm not giving advise, I don't know enough details to give advise, such as what your car is worth, what the salvage is worth and so on. Keep pluggin.