fixing water damage to the front of my trailer

[HoPInn]

I am in the process of fixing a corner of my TT that has had water damage and I think for a long time.
I took off the skin, then the Styrofoam board, then the insulation. The board that is damaged and needs replace looks like a 2x4 but is bi-leveled., I haven't seen that type of wood before. Its the first photo and its the front of the trailer that is holding the other parts of the frame.
My question, is should I look for this type or can I replace with just a 2x4? Also I have been reading and watching Youtube videos and some people use a jack to support the trailer when taking out the bottom frame work.
I am not sure if I need to do this or not..I don't want my trailer to collapse as I try to fix the floor and this front end board. (the left side of the trailer is damaged as well..the corner that holds the front and side together)
I am posting these photos to show you. I don't want to do this wrong.
One more thing, I am getting two different voices on what type of wood to use. Treated or untreated? what type of wood is the best? I was told today most dry kiln wood is pine. Thanks for any info..I know its a lot but I have a lot of questions. ***I dont know why this forum turns my photos sideways and I can't seem to figure out how to fix, but the last two photos should be turned to the right. ******

[plasma800]

dang!

[Tdub1966]

Wow..., I'm in the process of "remodeling" mine as well. Per your question if I understand correctly, my response is just to make sure when doing any type of work that involves the floor or frame is to use common sense and if it needs to be chocked or jacks or stands placed underneath, do it just to be sure. If your not doing anything with the frame and its already sitting on the ground and have your levelers down, I would say don't worry about anything and keep on trucking!
Ill try to send a couple pics of my adventure...