Thanks JimB! If that's the case, he must have replaced it with a "California King" mattress. To my knowledge, that's the only mattress longer than 80 inches (which is the length of standard "king size", either RV or household). The WIDTH dimension (70" versus 76") is what distinguishes an "RV King" from a "standard king".
Please let me know if I'm wrong, since I plan to order a 70" wide x 80" long custom mattress for my 3055RL next week, so it'll be done by early May.
Thanks!
Gary