where to buy new mattress

6" memory foam mattresses are available from Walmart.com.
I bought a Signature Sleep 6" for the bunk(trimmed it to fit the width), 8" Queen, and a 12" king for the home.
I hate spending a lot on mattresses.
The kids find them too firm for their home beds. Regular spring mattress for them.

Keyword in the OP's post was THIN.

Standard mattresses are NOT thin.

I can see where they're coming from, as space is a little tight in my overhead since I replaced the worthless original 5" thick camper mattress with an 8" thick household mattress. Inches count. I can't roll over without hitting an elbow on the ceiling.

We bought my parents a 12" queen size mattress from wayfair. It came in a box. They put it on the bed unwrapped it and it unrolled into a comfortable RV mattress. Only cost $250-$300 if I remember correctly. They have all kinds of mattresses. Thin ones like you want.

I took out the original mattress and put in Queen splits. Each half the size of the Queen. I can lift one and pull it out to put the mattress pad and fitted bottom sheet on it. The top sheet is a full Queen size sheet which is possible to fit unto the two halves. Better air circulation forward also.
The original mattress takes a crane to lift even the corners.

Hypervent:

http://hyperventonline.com/

Could someone tell me what hypervent is?

I have found these really good, we get the 12 inch ones.
mattress

If you go online, Walmart has a lot of choices. Our TC had an 11" mattress with a 2" topper when we bought it. Wife chose to down size. We bought a 6" Full size extra firm to replace the queen. Works great for us as we use the extra space at the sides and front for storage. Gaining about 6" of head room also made the TC feel much bigger.

Last year we changed our mattress in our Northern Lite 8-11 with
LUCID 5 Inch Gel Memory Foam Mattress - Dual-Layered - CertiPUR-US Certified - Firm Feel - Queen Size from Amazon $150.00 with Hypervent underneath .
The wife and I love it. The combination of mattress and Hyper vent came up to same height
as old mattress. Cabinet doors clear mattress.

I prefer sleeping on a plywood board. More support.