Hello and welcome. Anything you buy that's built in 1999 is going to be out of warranty and will need some TLC. National being out of business has no bearing on your 5th wheel. If the PO was kind to the unit you should be ok. As with any unit that old, things to watch out for are the water heater, fridge, furnace water pumps and the like. If you are only needing it for 6 months and are comfortable with how the unit looks, you should be ok. We've been full timing in our 05 Dolphin 5376 motor home for nearly 3 years. It's been more reliable than most cars.
Rick and Julia
2005 Dolphin 5376, W24, Solid Body Paint, Koni FSD's
2007 Wrangler Unlimited....it follows us everywhere.