Source for RV interior specs?

I don't know of any manufacturer that makes those type of specs public. Each RV built tends to be somewhat unique to meet specific design and price points. Dinette seats for example, can be used to enclose various electrical equipment, a water heater, a furnace, etc., depending on the floor plan and wiring/plumbing layouts. Ideally for your purposes, they would only be used for storage, as some are. If you're thinking of extending the kitchen counter into the dinette space, then of course you would first have to choose a unit with a suitable floor plan that has the kitchen counter and dinette on the same side. You would probably be better served to wait until you're closer to purchase time and start looking at suitable candidates to buy with your potential changes in mind. The likelihood of finding one that's exactly what you envisioned ahead of time are pretty slim, so be prepared to be flexible.

Your most reliable source would be a trip through a sales lot(s) with a tape measure. RV's are notorably variable in build dimensions, even between two rigs identical going down the production line together. They will always come out different, not where you can see, but in the details.

Look through some rigs and measure the ones you like .. that will give you a jumping off point for any mods you might like. Keep in mind the rig you buy might not favor what you want, but you'll have a ton of great ideas.