Trading in your vehicle will cost you dollars but the dealer makes it look like you just ripped them off. If the MSRP of a vehicle is 100K and you walk in with cash or will finance it's not unreasonable for them to discount it 20K. So you are paying 80K for the unit. Lets say you have a trade in valued at 20K if you sold it yourself. The dealer often says "I'll give you 20K trade in for your vehicle against the 100K price , you say cool. So you are still paying 80K for the vehicle but you just gave them your trade in basically for free.
When negotiating with a dealer you need to be informed. Go first without a trade in, make no mention of it. See where there numbers are, at how much they discount. Research and find out how much your trade is really worth if you sold it yourself. Armed with facts is where you want to be. Visit the dealer again and bring up the trade. See how fast their numbers change. IF you are ok with those numbers then go for it. Some people trade in to avoid the hassle. The dealer is going to send your unit to auction anyway. Get someone to help you if you feel intimidated by salesman, me I love buying new cars. I help family and friends negotiate and we make great deals.
Yes,dealers need to make money, but not tons of money. Also consider that you may need service from that dealer so you want a good relationship with them. Staying local is a good idea IF the numbers work. Independent RV repairmen is also a consideration.