how to figure the resale value

As I understand it, now that T@Bs are being titled, they should be in the NADA book.

Most areas have property tax on RV's that are computed from some list/guide so there must be some publication. Check with your local tax office to see what they use.

My insurance agent found it listed last Oct.

Is what they are listing the MFG suggested list price when new, or the avg sale price used? They seem pretty high to me if avg sale price used...

As a general rule of thumb, it appears that most Tabs sell for $1,000 for every year (i.e. 2018 = $18,000)give or take.  If you find one priced high, the seller is wanting the buyer to take the hit in depreciation.  As with all vehicles, and especially RV's, depreciation on a new unit is a killer.   Trade in value is brutal because a dealer won't give you a fraction of what it's worth.  Do a lot of research on used units through RV Trader, RVT, here on this site under "for sale", ebay, craigslist, etc.  Beware of sellers that are deeply discounting a unit, check it over very carefully.   NADA should start having more information on this as more units are sold & licensed.

@mfmccann, you express concern about the mechanicals, and those are standard items to check. The early Dutchman (Thor) t@bs were constructed from mostly wood. They are prone to water invasion and wood rot if not having been cared for religiously. Check under the benches for water stains. The same is true of inside/underneath the cabinets. Check the floor carefully for soft spots. Get underneath the trailer to look for problem spots as well. One poster here had the door side of the trailer separate from the body/roof as well due to water invasion and rot. Is the unit you are considering a clamshell?  If so, then check that the clamshell hatch has the protected hinge seam as water infiltration can cause the weight of the door to increase, thus causing failure of the shocks or even the clamshell hatch door to be replaced. If you are looking at a standard inside kitchen, look under the fridge for front end water damage. Check outside in front where the downward curve begins or at the junction of the diamond plate. Sometimes the evidence of water damage can be seen outside at the point where the front wall curve meets the floor. Look for swollen or bumpy areas under the diamond plate.or along the front outside floor joint.
That being said, there are bargains to be had if you shop carefully. There are quite a few early t@bs in good to great condition to be found.
Frames of the older t@bs were galvanized, and not a lot of problems have been reported. They have surge brakes, also not a lot of trouble reported. Something to consider is the trailer age and ease of finding replacement or maintenance parts and supplies. If you relish the challenge, then you'll enjoy the hunt, but if not you'll endure the frustration.
This is not said to discourage you, but just make you aware. Good luck. Examine with an informed and critical eye. Hope to soon be wishing you happy t@bbing. 

@mfmccann I’d resort to calling Elsie at Nücamp (probably just closed in Ohio) and asking her if they can source the parts or direct you to someone else. Her number is in the Nücamp contacts, Parts dept. in the categories to the left.