8 ft long by 6 inches thick is about the limit in size for most TT I've seen. Many are smaller. Would be about the same with a Class A with the door at the front like almost all diesel pushers. Some Class A rigs with the door in the middle of the living space might be able to go up to 9 ft long.
However, having a large item like that down the middle of the aisle is going to make using the RV for the several days of the trip a real PITA.
I wouldn't do it.
We carried two 8' kayaks on the inside of our older no-slide TT, though we had to pull them out every stop and store them under the trailer. Once had a CG manager complain about them making the CG look 'trashy'. So we left. I was never be able to use the TT for Wally docking when I had them with us.
Sold them when we got the new bigger trailer because the door/wall/sink doesn't allow enough room to get them inside - and because with the slides pulled in, I don't have any room to carry them.