Our older pup joins us in bed for a short while then jumps down to enjoy her own nest - which is in the front closet where a washer/dryer would fit in our Montana. The young one will sleep in her crate until she 'remembers' her manners.
As far as a king bed in a 26 foot 5er - you will be losing way too much valuable living space. A regular queen might work at 80x60 if you have a standard double (hopefully not one of the way too short RV doubles)
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')