We purchased our fifth wheel from Terry Town and their package wasn't much. Battery and propane were included, but the sewer hose was a short piece of....junk. Prices in their "store" we're high. Fortunately we are had all we needed from our previous RV.
I think you are right. Do some comparison shopping and buy good quality sewer hoses, fresh water hoses, tank treatment (your choice), etc. before your first camp trip.
You might talk them into giving you credit for the "value" of what you don't need to spend on other stuff in their store. We bought ours 2 years ago and memory says they told us the included package was worth $300+. HA!! We buy our batteries for about $100 each and propane costs vary, but highest I've seen is $3.25 per "gallon". So you can do the math to figure what they are charging for the rest.
If you can, have someone video the entire walk through. It's amazing how many things are forgotten within the first 24 hours. Took me days to remember what switches operated what lights