It is probably a King size bed instead of a California King. Regular kings are shorter but wider than California Kings. Both of our recent motor homes had that size bed. We recently swapped out the regular King on our current coach for a new mattress from the Sleep Train, and a new regular King worked fine. The only concern we had (and I say concern because it worked out fine) was as we were pulling in the slide, we realized how closely it would fit as it did not have that extra cut in curve that many motor home mattresses have on the foot end. In addition, it is a much better quality mattress than the one that came with the coach, and it was a lot thicker. Consequently, while I am short enough not to notice, hubby barely fits sitting up in bed under the large overhead cabinet above the head of the bed. You can get a mattress with the curves and thinner at Camping World, but we chose a high quality mattress and it works fine. It made an enormous difference to us in the quality of sleep we get on the road!
Just measure before you buy. And Sleep Train will deliver it right into your coach. You do get some odd looks from the delivery guys, though.