Transmission wont go into rev/drive

[AZmathteacher]

2003 International 3800 with Allison transmission

Backstory: I had the bus parked for a few months while I was out of town. Came back to rats, mice and a ground squirrel using my bus as their new hotel. Engine area filled with twigs and nesting - no visible chewing of wires. I cleaned it out as best as I could and took it on a short trip. No issues on the trip at all.

Once I got back, I tried moving it in the middle of the night because the emergency roof exit ripped off on the highway and it started to rain. I needed to get it in my driveway and throw a tarp over the top. I didn't let it warm up or anything and put it into drive - boom the truck knocked so damn hard. I got it to my drive way with no issues. After that night the bus was stuck in 1st gear - knocking super hard if I put it in reverse or drive.

I didnt use it really other than moving it back to a spot on the side of my house. I checked the transmission fluids and there was none in it which is weird because I had my mechanic look over everything when I bought it and didnt get more than 800 miles on it before this happened.

I did have a rat chew through some stuff in my kia which made it stuck in 4th gear and it wasnt the actual transmission. I'm hoping this is my problem right now. After getting tranny fluid in it and letting it run, I took it for a spin. No knocking when I put into reverse or drive BUT it wouldnt leave 1st gear. Now I cant get it out of neutral. It feels like you are in reverse or 1st but it doesnt go anywhere. You can feel the smallest pull like it starting to move but doesnt have the power to turn the wheel.

Am I completely fucked? Could it be something as simple as a rat chewed through a wire or does it sound more like I blew the tranny? Any help would be much appreciated. Cheers!