A Lippert mattress? Owee wowee, just thinking about that makes me want to reach for some pain killers... Sleeping on a bed of stones would be better. :E We got rid of our OEM mattress after sleeping on it for just a few days.
There are many brands, versions and qualities of memory foam mattresses on the market. We bought an 8" one that was on sale at a mattress shop that we happened to drive past while away on a weekend camping trip. Has been excellent and gives a better sleep than our fancy expensive one at home. If price isn't the important factor and want something of better design & quality, some googling may help. This
website has some good info. There are even 3 types of memory foam - "traditional", gel-foam type and plant-based.
Unless you extend your bed platform or happen to have a full queen size mattress already, you may need to shorten a regular queen foam mattress to "short queen" length. Easy to do with a sharp knife or bread knife. Also note that the memory foam mattresses are heavier and can throw off the balance of the gas struts on under-bed storage and you may to relocate the mounting points.
If we were to ever get a new mattress for our TT, I'd make the platform an inch or two longer. A short queen is a tad too short for me and I hate it when my toes hang over the end and have to wiggle back up to the headboard. Annoying!