Secondly, in 1978 the longest trailer was a '31 foot which is measured from bumper to ball - it may have been 31' plus some inches, but they were called 31'. There were two distinctly different models of 31-footers the Excella 500
and the Sovereign
. The Excella being the upgraded model. Within each of those finish and feature packages, there were various layouts (twins vs doubles, rear bath vs center bath, etc.) so you may want to look at the two links to see if something strikes a familiar chord with you.
As far as it's sitting for a several years goes, these trailers are made to travel - not sit. The least you would want to do before attempting to travel with it is:
1) change the tires if they are older than 5 years old
2) repack the wheel bearings
3) then take it somewhere for a "once over" if you aren't comfortable checking things out yourself - major safety systems like the propane lines, propane appliances & brakes should definately be looked at & tested before using them
You may also find Table C on this Price vs Condition
website handy in determining the cost to repair some of the deficiencies. Regardless, the $2000 asking price seems to be more than fair...
Good luck if you decide to go for it!