It almost sounds like you are looking at a 24' rear entrance (entry door to rear of axles) from the mid-1970s. This was a relatively unsual trailer - - it seems to be somewhat less common than the front entry that has a floorplan resembling the Airstream TradeWind.
Even if it isn't one of the rear entrance 24's, it could still be counted among some of the other less common Argosys. It could be a Minuet from '77, '78, or '79. The Minuet is an interesting trailer due to the fact that it is 6" narrower than the standard Argosys and was available in three lengths (20', 22', or 24') with the two shorter versions being single axle and the larger being a tandem. In addition, the Minuet often featured experimental features found only in the Minuet - - for instance a number of the 6.0 Metre (20') rigs have an aluminum composite floor - - so they avoide the dreaded wood rot, but gain the question of what floor covering to use if you don't want carpeting.
If you find that all systems are working and the floor is solid, the price you mention is well within the averages. Just be prepared that even if all systems are operational at this time that they may begin failing at any time - - my Argosy has been fairly typical in that regard - - in the past year (since purchase) it has needed a new water heater, Univolt, and water pump. The major expense - - the refrigerator continues to operate, but I am prepared that it too will eventually wear out and need replacement.
You should be able to get most of the information about the trailer from the data plate that is attached to the front panel (behind and to the side of the propane tanks). The VIN number should be arranged something along the following lines:
20 = the length measured in feet
D = bed arrangement (D=Doubble/T=Twin)
8 = last digit of model year (1978)
V = Versailles, OH assembly plant
1666 = Sequential production number
For more information on the Minuet thread, see:
Good luck with your investigation!