We replaced the RV queen in ours with a standard queen memory foam. Never even took the plastic off the original mattress, replaced it before we ever slept in it. The head of our bed sits over the outside storage compartment and the foot lifts up for storage inside. We just pulled the plywood support and replaced it with a larger piece, no extra supports or anything else. I'm not sure how thick your topper is, but I'd be worried about the transition from mattress to wooden box right where your head lays being uncomfortable.
Pluses:
* obviously longer mattress
* easier to get a standard size replacement
* easier to get standard sheets that fit correctly
* new mattress is 1000% more comfortable
Minuses:
* Less room to walk between our dresser and the foot of the bed
* The gas struts will not support the heavier mattress. We could replace them, but so far we're just using a length of 1x2 to hold the platform when we need it held open.
* New mattress is significantly taller than old mattress. Extra thickness makes it more comfortable but harder to get into.
* New mattress is heavier than old one.
If we ever replace it, I think I will go back to a slightly thinner mattress. I think the wife ordered a 10" thick and I believe 8" is standard. Other than that, we couldn't be happier. We will never sleep on a standard RV mattress again if there is any way to avoid it.