We have had this 
Spa Sensastions Gel Memory Foam Mattress in our last two RV's.  First, we had a TT with a wall at the bottom of the mattress and only 77" of room.  It fit perfectly, but did make the mattress a little bit snug when we were putting sheets on it.  Now in our 5'er, there is nothing at the end, so the mattress hangs over about 4" and it works fine.
We spend 100+ nights in our RV, so we must have a decent mattress and I ordered the mattress before we even took possession of our 5'er.  It was waiting (in the box) for us to get the RV home.