HELP support stringers to bulkhead repair under water tanks

[BluewaveRV]

I have two (so far as I can see) stringers that the welds to the bulkhead, a foot infront of the rear axel, welds are broken & they are bent down.
No shop has a pit to drive it over not a hydraulic lift able to get access to this area near me.
Looking for IDEAs of lifting the rear of RV safely to get under it. I want to use "L" shaped gussets to rivet/bolt on to stringer & bulkhead. I fear there in not enough material to just weld & to weld or weld a gusset on the heat & plastic 100, 60,40 gallon tanks could be in jeopardy. The are of the broken welds looks to be will rusted.

[GBRG07]

I had to rebuild the steel structure in the wet bay on our Coach. I did it on the ground, I have some home made ramps that I put the rear wheels on to give me enough room to get under and enough room to work.
I made the ramps from 3/4 plywood, they are 24 inch's wide and about 8 or 9 inches tall. They are solid all screwed and glued together.
I had a similar issue as you I think but the more I poked around I realized I was going to have to pull the fresh water tank, plumbing and all the plywood. Was a huge job, thankfully I have welding and fab tools.