Salsa, I'm afraid I don't have much in good news for you. Your "bought a queen size blow up mattress that seems to do the trick for the time being" statement probably set the old, feeble, bad-back, or just plain fussy people back ten years. Most of us would look at a blow-up mattress and immediately be on the phone with Holiday Inn Express.
I can assure you that more than half the RV's you see in parks and on the highway have a mattress that is inferior and an odd size.
And the news gets worse. Where would you ever find a custom size king/queen mattress for $700? There are camping people who doctor up their crappy mattresses with toppers and have some luck, but you threw yours away so you have nowhere to start. We shopped at a custom mattress factory in West Palm Beach and the top of the line for our RV (king) was $2100. Nope, you are correct, we are still sleeping on the piece of junk from the factory.
See? I told you I had no good news. And once you do find a solution you'll log back on here whining about buying sheets!
God, how I love RV'ing!