A lot of travel trailers the serial number is stamped in the side of the frame rail , just ahead of the body , under the battery & propane tan mounts.
The info should also appear on a weight sticker in the back of a cupboard, like the picture below.
A tire inflation decal should also have the unit's production date.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.