Is there never any under-dinette storage?

My Arctic Fox used 25 percent of the space for the heater and the rest was storage. My Host uses 75 percent for the heater and half of the black tank leaving 25 percent for storage.

I have/had 2 campers, both with 6" dinette in slide and both had big storage under the seats.
The rear was divided where I used exterior for hoses and other camper stuff, while inner was used for dishes.
Front compartment was pass-thru and we could really overlaod it with dry food and sodas.

You owe it to yourself to go look at a few campers in person, or even online rathe than the barrage of questions that have seemingly obvious answers.
To your question, most TCs DO have under dinette storage. And/or an appliance mounted there, typically the furnace.
You can be rest assured there isn't much space in a TC that is "wasted."

My 1055 has the dinette in the passenger rear corner. Under the back bench is the outside rear compartment where I can store a Honda 2000 water hoses, filters, electrical cables. Under the front bench is a cabinet with two shelves. The lower shelf can only be accessed from inside, the upper shelf is also accessible from the inside as well as an outside hatch.

I have the model in my signature. Do a search, and you can find lots of pictures.

What models do you guys have? Pictures?

I have one sliding drawer and a small cabinet on the two ends.
And I have one door that gives me access to the inside of the truck bed.
Down the long section my bed rails are under the dinette.

Ours have sliding drawers. It all depends on the RV, some have furnaces or electronics under one side. Probably has a lot to do with whether it is a basement model or not.