On the NADA "M" lettering, that is a NADA thing for reasons not understood. NADA uses the M on every camper, motorhome, 5th wheel, travel trailer or truck camper. Sunline used M for motorhome, F for 5th wheel, T for travel trailer in most cases, and TC for a truck camper. Nada never used those, they just put an M in front of everything. The good news, is the model number really does not cross over between 5th wheels, travel trailers, truck campers, etc, so just use the M- number they have listed for your camper. In your case, it would be M-24A.
Remember on the NADA site not to add extra value for things that came as standard from Sunline. All the standard items are included in the base price.
That said, with Covid pricing right now, NADA values may not be that accurate and campers are selling for lots more than pre-covid. You can use NADA as a place to start, but check other sites selling used campers of similar size and age to get a closer selling price. And camper condition plays a large roll in pricing. It is amazing what poor condition, water infected campers are selling for now to unsuspecting first-time camper buyers.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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