Hi and congrats to the new rig
After several years on RV Mattress with a Topper, then a "RV Mattress" we went finally to Mattress Firm and took the model we liked most.
We prefer a full Memory foam mattress so the choices are limited already.
On our actual Montana 3123 we had to push the true king mattress quite a bit into the slide but it works.
I think the slide is 2 inch to narrow but that's totally no issue.
Mike