The NADA "blue book", which is actually yellow with blue writing, has a version for RVs. The value of any RV over about 10 years old depends on condition more than model year. Your TT is 29 years old, so any "blue book" number is going to be almost useless. The model number indicates interior length, so your 31' TT is probably a model number 28-something. So just for grins, here's the numbers per NADAguide:
1983 Nomad M-2850A,B
Suggested List Price . Low Retail . Average Retail
Options: (add options)
110 Volt - 13,500 BTU (Roof Mount) $85 $100
5 cu. ft. Refrigerator . $75 ........ $90
Flushing Toilet/Holding Tank . $75 ..... $90
Waste Water Holding Tank . $75 . $90
Water Heater 6 Gallon Gas/Elec . $30 . $35
Awning 15' . $115 . $140
TOTAL PRICE . $10,709 . $1,115 . $1,345
Plus add more for other options such as microwave, sterio, upgraded mattress, etc.
1983 Nomad M-2850A,B Standard Equipment, Prices & Specs - NADAguides
If your TT is in good shape with tires less than 3 years old, and all the appliances work good, then it's probably worth what you paid for it. But the typical 29-year old TT is worth less than that.