One night... thats right. We slept on the mattress that came with our new RV for exactly one night before we spent $150 for a mattress topper. This helped a lot, but was not the final solution. Later that summer, we were able to buy a Denver mattress on huge discount at a RV rally, so we opted for that.
There are several different Denver mattress available, with the typical hype that many products have. I don't know if Denver is the best or not, but I suspect that some of the "lower end" Denver mattresses are made just so that RV companies can advertise they ship their product with a Denver mattress.
We obtained a 11" thick Euro Foam Supreme mattress. You can buy thinner mattresses, but I would recommend the 11" as they are very nice. When I measured for the mattress, I did make a mistake and ordered a Camper Queen, or Short Queen mattress, while the old mattress was a full Queen. As it turned out, it worked out OK as the Camper Queen is 5" shorter, and given that the platform was quite a bit smaller than the original mattress, it sagged significantly. The new mattress fits the platform perfectly.
And having 5" of additional space at the foot of the bed is not bad either. Even though I am 6'2" (74"), I found the 75" of the Short Queen adequate.
So you will want to make sure the size of your existing mattress when ordering a new on by using the chart above. Another option for some sizes of Denver Mattresses are rounded corners.
The mattress was delivered via UPS, rolled up in a box. The mattress itself was shrink wrapped to make it easier to handle. One word of advice - when you remove the shrink wrap, the mattress will inflate like a dinghy, so get the mattress in place before doing so. Things will be so much easier that way.
It actually took several days for the mattress to reach the 11" thickness, but it is sooo comfortable compared to the previous mattress. This is probably the #1 improvement I would recommend for your RV, especially if your old mattress was as bad as mine was.
In reality, it would have been better for the RV manufacturer to just not include a mattress. The $100 or so it would save could be applied to a new mattress. Afterall, RV's typically do not include a battery from the manufacturer (installed by the dealer), so why not the mattress?