Getting mixed information on mattress size...

[PaintballMomma]

I’m “buying” a 3124, the owner is telling me I absolutely have to have a 76” sq mattress of 4” thickness, which doesn’t seem to exist. So can I just buy the 6” foam king size mattress I found on Amazon or not? (I’ll also start reading/searching through the forum, but I’m new to that to so...)

I’m actually being gifted the RV sight unseen, but am told I need a mattress and tires immediately...so I’m bringing those with me from Illinois...RV is in Florida...

Thanks for any advice/help,
C