All the older Dynasty models seem to be tagged wrong in NADA online. Strange.
Since you are looking at the online NADA RV Guide, you should know that it is based on average depreciation, which becomes totally meaningless after 12 or so years. After awhile, condition is everything.
Since they merely depreciated the original selling price, I doubt if tagging it as gasoline had any effect, but it might dissuade a prospective buyer if he looked it up. To the best of my knowledge, there were no gas-powered Dynastys in 1998. Or ever.
I would find some other sources for market value pricing. PPL Motorhomes site lists actual selling prices for everything they have sold(on consignment) in the last two years - that's a good place to start.
Used Diesel Motor Homes Sold - PPL Motor Homes
I see a 1997 Dynasty went for $31,000 back in February. A slide (or not) would make a big difference in price, I think. And private sale prices are typically lower than dealer sales, which usually have a trade-in fudge factor in the price..