Sounds like you need to pull down the underbelly material to see what's going on. Not a fun job and just had to do it on our Spree in a CG after the toilet overflowed during the night.
There are a few screws into some of the trusses going between the frame's I-beams. There are some screws at the front and rear that are installed from above. The coroplast otherwise just sits on the lower flange of the I-beams. You will need to remove some of the foam around pipes and wires that penetrate the coroplast. you need to carefully bend the coroplast downwards in the middle to be able to get it off the I-beams and may need to slide it forward or rearward to get it away from the stab. jacks. May be 2 - 4 pieces. You'll need some Gorilla tape or RV Scrim tape and some gray landscape expanding foam. Be careful not to crease the coroplast anywhere.
Some people have installed cross-pieces using steel or aluminum (right angle cross-section) for added support of the coroplast.
If you do take the coroplast down, you will have a rare opportunity to fix up the poor workmanship in wiring & plumbing and maybe some other things (like insulation?). When I had the coroplast down, I ran a new #6 wire from the A-frame to the converter for an AGM battery in a space behind the shower. Battery in the A-frame is gone. Installing pipe insulation on the PEX lines might be a good idea.
The highly touted "heated underbelly" in TTs is a joke the way it is designed and it's a wonder it works at all. You might consider improvements to it. There are two 2" ducts from our furnace that run into the rear of our underbelly and I doubt there's any heat getting to the FW tank in the front. They install a layer of batt insulation on top of the coroplast which is not a good choice for the location. It also gets squished under the tanks which reduces R-value. If you have the time and inclination, could install rigid foam instead.
Sure not impressed with KZ's heating design on ours. The main 4" duct runs under the floor and air coming out was rather cold. I pulled the duct out and installed an insulated one. The duct to the bedroom was only 2" and I upgraded it to 4". Both mods were a huge improvement. In a previous Spree, they had the main outlet grille under the dinette table and the return air grill inches away. Not very efficient... Our AC ducting was also poorly installed and had to fix it up.
We have had many many workmanship issues in our Spree. Poor quality control for sure. Would not be surprised if your issue is also poor workmanship. Our KZ dealer was hopeless at fixing things. Have you tried contacting the factory? I used to be able to talk to someone there but don't know what they're like now since Thor took over.
