Night Therapy 13" Medium-Plush Gel-Infused Memory Foam Mattress

Zinus Spa Sensations 13" memory foam mattress at Walmart. Was about $350 when we got it years ago, probably has gone up in price but would still be under your budget

Ours came with an rv king but we put a regular zinus memory foam king and it fit fine. A bit longer at the feet but doable. And way more comfortable.

Bro I bought a Zinus foam mattress almost 9 years ago and it's the best mattress I ever bought for $200 shipped.