"Agreed Value" insurance is not typical RV or automobile insurance. It is provided by some insurance companies for vintage vehicles and in this case RVs. It usually involves having a professional appraisal done before the insurance is offered. This appraisal establishes an estimate of vehicle worth with proof of fair market value of similar trailers. You typically provide pictures and receipts of the work done to the appraiser. They will provide you with a multipage booklet with photos and text proving the value they assess. That booklet will be what you provide to the insurance company to establish "agreed value".
Sorry for your loss and hope you can negotiate as much as you can from your insurance company!
1965 Airstream Caravel, 2003 Ford F150 SuperCrew, 2003 Miata Special Edition, 1993 Miata Black & Tan