I traded in a truck related to my business for a new one in 2018. TT asks for Sales Price of old vehicle. Is this the trade in value? Is that used to determine gain?

like kind exchanges (deferral of gain on disposition) for vehicles (and everything else but real estate) has been eliminated for 2018.  you are deemed to have sold your old vehicle for fair market value.

is trade-in value FMV  - who knows.  dealers have been known to play games with this. .  but that's what I would use or you could look up blue book value.  any change you make to trade-in value would change the cost of the new vehicle.

gave you an example of what dealers may do

say a vehicle with MSRP of $52K usually sells for $48k . your trade in has a real FMV of $6k.   the result is that  the dealer would want $42k net.      but say you want $7k for the trade in,  then the dealer would sell you the new vehicle for $49k give you $7k for the trade in .  but you still would owe the dealer the same $42K net

worked in that industry for many years.