You can physically sleep on some thick foam toppers, but a topper is not generally designed to replace a complete RV mattress. If the existing mattress has failed or there is no supportive base, compare a replacement mattress rather than assuming more topper thickness will solve it.
A topper and mattress have different jobs
A topper changes the feel of the surface below it. It can add softness, pressure relief or a different temperature feel, but it normally relies on the mattress underneath for most of the support.
If there is no supportive mattress or platform system, the topper has to perform a job it was not necessarily designed to do.
A failed mattress can still feel failed under more foam
Deep body impressions, collapsed foam and poor edge support can carry through a topper. A thicker comfort layer may hide the problem temporarily without restoring the underlying structure.
If the bed is sagging badly, compare the cost and fit of a replacement RV mattress before stacking more material.
RV dimensions can make replacement less straightforward
Short queens, corner beds, radius corners and hybrid bunks may not match residential mattress sizes. That is one reason owners reach for toppers in the first place.
Measure the actual platform and mattress before deciding whether a custom-size replacement or a carefully fitted topper is the better value.
Do not ignore airflow underneath
Whether you keep the old mattress, replace it or use a thick foam layer, moisture can still collect over a cold RV platform.
Keep under-mattress airflow and condensation as a separate fit question.
Quick answers.
Can I sleep directly on a mattress topper in my RV?
Some thick foam toppers can be slept on, but they may not provide the support, durability or fit expected from a complete mattress.
Should I buy a topper or replace my RV mattress?
A topper makes sense when the mattress is structurally sound but uncomfortable. Replacement becomes more reasonable when the mattress is badly sagging, damaged or unable to provide support.
Can I put an RV mattress topper directly on plywood?
That can create support and moisture issues. If you are considering a foam-only sleep system, verify the product use, support needs, ventilation and platform conditions first.
Use your saved trailer context.
Trailerstead can help separate a comfort-layer purchase from a mattress-replacement decision for the bed in your trailer.