King size fitted sheets

Amazon has these for king size RV mattresses.
Clicky

And as a thought, I think MH mattresses are too thin so I put a 2" foam topper on them. That would add to your height to maybe make your new sheets fit better.

Bill

I saw once where a guy was using suspenders to hold his sheets in place

old guy wrote:
I saw once where a guy was using suspenders to hold his sheets in place

Those are actual sheet clips and they work great.

I know this doesn't address all your concerns, but some RV suppliers use oddballs. King is 76" x 80", Calif. King 72" x 84". I have read about other "King" (European) sizes. You could state your case to the seller and tell them you want a standard King or Calif. King. Use other RV's as an example.
I've never been comfortable with the RV supplied mattresses...they are the first things to go...went to a Numbers air bed.
Suspenders will work.
I see from the Trackrig link there are RV Kings 72" x 80"...wish they would state the seam length.

We used the sheet clips as mentioned above until we put a 2" memory foam topper on the bed, now they fit perfectly.

You will probably lean real soon that that mattress is junk. Live with what you got u til such time as you figure that out and get a real good bed. Then your problems will be over. At least sleeping!

They are actually suspenders. We've been using them for years. We have a 3" memory foam on top of the mattress and we still need the suspenders. I purchased a pair of men's suspenders at Wal Mart, spread them out in an X pattern under the mattress. Clip to the sheets and it keeps them tight.

Our King is 72x74 short. I bought jersey sheets from target and they work great. Get the cotton ploy blend and they stretch to a perfect fit. I bought queen size sheets.

donn0128 wrote:
You will probably lean real soon that that mattress is junk. Live with what you got u til such time as you figure that out and get a real good bed. Then your problems will be over. At least sleeping!

LOL!!!

HOW TRUE!

First three trips in our Copper Canyon, DW had back aches and head aches! We made measured, and had true queen. We went to the mattress store and found one we liked with the correct firmness, has not been an issues since. We buy standard queen sheets, no issues, and we sleep like rocks.

We bought a very nice set of fitted sheets for our standard queen bed at home, from a discount store, at a steal of a price. Got the set home only to discover the sheets were too short! No return, of course. But when we tried them on the queen bed in the MH, they were a perfect fit! Bonus!!