Regarding the replacement generator.
Check to see if the "compartment" is an actual compartment or if the floor is nothing but the base (part of) the exiting generator. If so, check to see how much below the existing generator the new one will hang once you make your own bracket and shelf.
Go examine the proposed generator and see how much height is taken up by the base and wheels. If mounting inside the RV, there's no need for the wheels and base to be attached. You can keep those as a platform for if or when you remove the generator and want to roll it around.
Next remove the side covers and see what is above the working portion. Often the top is just an integrated handle and fuel tank. If the handles can be removed or eliminated, you may gain another inch or so. The fuel tank may also be removed or relocated within the compartment if you can find a way to ensure fuel delivery.
You should be able to connect the generator to RV just like the previous generator without he need for added or outside cables. If you want to keep that replacement generator intact as is, a cargo carrier setup is very common. Just know you'll have to do some creative cabling if you plan on using it while in motion.
