You must be able to get a 60x75 RV queen in there. They are flexible since there is no box to worry about.
Any of the RV mattress from Rocky Mtn and MattressInsider will fit. They ship compressed and you lay them out on the bedframe to expand.
We are going the other way and converting our queen to twins.
2008 Classic 25fb "Silver Mistress"
2015 Ram 2500 6.7L Cummins. Crew Cab, 4x4, Silver
2010 Tundra 5.7L, Double Cab, 2wd, White