We bought a mattress topper for our TT but, as we also like to travel with our duvet etc on the bed, found we couldn't close it down.
We've decided on Duvalay mattress toppers (2 x singles), which we can roll up easily (quicker and easier then deflating and rolling up SIMs) and just roll out when we get there and set up. They'll take up no more room than our SIMs (which we have for tent camping) and you can store them with fitted sheets already on, so just roll them out and re-position the duvet on top.
At 70 yrs old my wife and I use 2 x double SIMs and 1 x memory foam topper on the useless mattress of our Cabanon Mercury TT.Its advertised as a luxury item but it felt as if sleeping direct on wood! Both roll up and store in cupboards when not in use. Lovely tent and had it since new. Don't believe Cabanon's stated time for erecting of 15 minutes, it takes us an hour, and always have done since new Had to modify cooker legs to take wheels as it is heavy to carry. Generally a good quality spacious TT
We've had Cabanon TTs previously (several different models, over the years) but now have a Raclet Moovea. The bed certainly is more comfy (the mattress is much better quality than on the Cabanon and Conway type 2+2 TTs and is supported by slats rather than by a bed board) but I still do prefer to use a Duvalay topper as well. Then again, we also use a memory foam topper on our bed at home, and used to use Thermarest Dreamtime XL SIMs (which include a memory foam topper layer) in our tents, so maybe it's just that anything else now feels like a compromose?
I use a SIM in our Raclet Solena 2015.
However???? Why do you need to blow into a "Self Inflating Mattress".
OK, the question is slightly tongue-in-Cheek, but are we being sold a Pup?
Don't know if you have sorted this,but we take a 4" memory foam topper for our Pathfinder. We lie it on top of the seating area with the toilet door sitting on the top of it. Folding down is no problem,there is still room between it and the bed.