I hate to bear bad news but most campers until fairly recently did not have 'manuals'. The idea always has been that the dealer walked you through the use of all the items. (along with recommending any service be done at their shop)
I'd contact Lance and see if they have any historical marketing flyers available for that year or near that model year. RV appliances have not changed much over the years so surfing by different OEM websites may bring up a similar manual for the appliances that are installed.
Last but not least all campers that cover the trucks tail lights will have ether a outlet or cord to be used as a 'umbilical cord' electrical attachment between the truck and camper.
- Mark0.