Pool, I would guess your policy is an ACV (actual cash value) policy. Most policies that provide replacement cost only do that for the first few model years but you never know till you read the policy for. You could also have an agreed value. It my bet would be ACV.
how Allstate determines their value could be simply looking at a book value. If I have a rig totaled I would look at Nada (they have a pretty good detailed way to determine value) but the one thing I don't like with Nada is they don't have an adjustment by Zip Code. After I print off that I would then look at RV Trader or any online classified listing. I would first look for your spherical rig. You might have to do a long range search to get compatibles . If you don't find enough compatibles I'd look for similar models. Keep copies of what you find for when Allstate gives you a figure.
Bottom line, they should have a value that represents what you can buy the same year, make, model with options and in the same condition, minus your comprehensive deductible.