assume kitchen is in the slide? if so then the reason Thor didn't bring cabinet support to floor is to allow drain hose, electrical, water lines and propane line to slide across floor without obstruction during slide in/out. While it is important to keep this area clear so things can slide, Thor has provided a lot more space than needed by removing all cabinet support to floor. Not good.
To fix, first, get a crowbar and some blocks under the bottom rear cabinet frame support and lift it back up where it belongs. You'll need all the drawers out for access and maneuvering. Second, dead center of back horizontal support mount a flat board (8x10) vertically to screw into cabinet support and floor. Use some cheap L brackets to screw to floor. This will keep drawers horizontal and will raise countertop back to meet backsplash bottom. Third, get some cheap shelf supports that have 6" to 8" legs. I like the ones that have long triangle shaped legs. Put one leg flat to floor in corners of back cabinet support. Screw to floor and to cabinet frame. Fourth, use some of those L brackets along side framing to attach to whatever you can find. Be careful of screwing into veneer as thin wood won't hold much. Fifth, caulk along backsplash. Use crystal clear so it will disappear. Sixth, after you've trued the whole mess, you may need to adjust the drawer glides too. Just remove rear screws on the glides, level and screw into new holes. You want the drawer fronts to fit flush with frame.
Biggest problem is deep drawer is not designed to carry a lot of weight. Too much weight in this drawer will pull all of framing down...with countertop following.
As long as you have the drawers out, flip them over and glue all the joints. You will probably find no glue, just staples, which won't hold the weight.
And check that really unique velcro bag around pipe, electrical, etc. that is supposed to seal out cold air. You might find it is wide open. Mine was.
Did mine a few months back. All fine since.