I'm not familiar with the Heritage Glen line, but one thing I always do when I'm considering a new trailer is check NADA. Banks will use this information when deciding how much they will loan. The model you are looking at is here: https://www.nadaguides.com/RVs/2018/...6583096/Values
So it looks like the dealer is selling it below current value, if they are asking in the $18-19,000 price range. You won't take a huge hit in depreciation like most new trailer buyers do.
For comparison, I bought my 2015 Bullet 220RBI new in August of 2015 for 25% off MSRP, $4000 below the price posted in the trailer.
Things to consider when purchasing a leftover model: do the original manufacturer's warranties start at the time of purchase (i.e. do you still have a 12 year warranty on the roof, or do you only have 10)? Has the roof been periodically inspected and maintained by the dealer while it has been sitting on the lot? What is the condition of the sidewalls and graphics? The tires may already be two years old, you will want to replace them in the next three years, regardless of warranty and tread condition. The date of manufacture is on the sidewall of the tire, on a 2018 trailer the tires could have been produced in 2017. Is there any checking of the tire sidewalls? This is more likely to occur if the tires have not been kept properly inflated. In other words, it is the exposure of the exterior of the trailer to the sun that warrants closest inspection.
Finally, are all of the original hang tags and stickers on the various components inside intact? If not, ask if the the trailer has been used for personal use by the owner or a member of the staff (more common at small dealerships).