Hi Chopper Bill,
Before leaving on our 2 month summer trip, we replaced our original mattress with a 12" memory foam mattress from
Lasting Impressions. The RV King size is $499 delivered (2-3 day delivery included). It's 72" wide but it compressed a bit and fit right in even though the spacing at the head of the bed is more like 69 1/2" I think. We had previously put down tiles to let the mattress slide more easily and this mattress has no problem fitting the space.
They come in 8", 10" and 12" thickness. The top 3" is memory foam and the rest is conventional foam. The thicker base is recommended for heavier people like me. They said a 10" would work for me at 250#, but the 12" would wear better over time. I measured the clearance between platform and window treatments and 12" is tight, but fits. The head of our mattress sits higher than normal because we have a 4" wedge at the head of the bed to help with acid reflux. So we have to use a little care when moving the mattress into place.
We sleep
really well on this mattress - so well that we're now ready to retire our very expensive coil-on-coil mattress at home. The only downside I would mention is that I compress the foam underneath me overnight and have to roll a bit to get out of bed. Not a bad price to pay for a great night's sleep.