I'm going to be the negative voice here, sorry.
The plan of marrying another frame rail under the existing rail will work but you're getting into a fairly big rebuild at that point. Plus, you're relying on the old thin/rusted rail now between the new rail and the trailer body. It's more of a patch than a repair.
As for the axle, yes you can switch that axle so that it sits under the springs and gain a little lift. Welding spring perches on the other side of the axle tube is easy and you may even be able to cut the current ones off and re-use them. However, it may be cheaper in the long run to simply replace that drop axle with a new complete straight axle. You'll gain the clearance you desire and get new brakes, bearings, spindles, etc. in the process.
Before I started buying parts, materials, welding frame rails and re-positioning axles, I would step back and make a seriously honest evaluation of the overall condition of the trailer.
I think the more problems you look for - the more you will find.
First off I appreciate a negative voice, it gets me questioning the project piecemeal and in its entirety. That’s exactly why I posted here so different view points would help me evaluate what I’m getting into. I’m naturally a optimistic person and have turned more then a few sows ears into if not a silk purse, the a useable bag. So criticism is needed to help me out.
First off I am capable of doing the entire job start to finish. But being able to step back and evaluate the end product is hard. Hard to see the forest for the trees.
What I’m looking for is a camper that is secure, dry and can be heated. If the fridge doesn’t work then there’s a cooler. I’m simply looking to be a step up from tent camping.
That being said, I have to do this cheap because I’m not real bucks up. I’ve looked at a lot of under $4000 travel trailers for sale up here and they all need work just to use.
This trailer has a good tight roof as testament to sitting out in the weather & snow load for all of the last 20 years not having any leaks. The 12 volt wiring has to be replaced, 110v system is very basic and in good shape, the plumbing system is ok, the tank is probably needing to be replaced. The furnace is an unknown at this point. The fridge looks to just need a gasket, the old one is so hard the door just won’t quite close as opposed to being sprung as I first thought. There’s a lot of work to be done if it’s going to be useable for sure. I’m not looking for a show piece here, just a usable camper.
You see I’m talking myself into it.
Yeah P&P I have replaced axels on travel trailers before and the thought was to just buy a new straight axel complete with new brakes. Slip in gain the height need for this country. But the old axel has to come out anyway so I’m want to assess the brakes and decide then. One idea I had, and I did this to a 1974 brand new tandem axel travel trailer was to flip the axels under they were straight axels and the existing spring perches were good just by rolling the axels over. Proceeded to drag it 4000 miles up here.
So I was thinking that 2 $10 spring perches could be simply laid on top of the axel and the clamping action of a new u-bolt assembly would hold the axel in place without welding. I don’t weld so I’d have to take it to a shop. The perches are a 180* radius so they would nest against the old ones. The brakes I will pull open today and see what’s there. I believe the axel is the cheap part. A replacement straight axel with brakes is close to $500. If I have to completely replace the brakes, backing plate and all on this axel, is about $200.
The frame on the other hand is my dilemma. Yes it’s rusty and crusty but it’s towable as is but a failure is in the future I’m sure. It’s not bent but moving from the house box forward. Joining a lower heavier box tube under just makes since to me. Challenge is going all the way back to the springs of the axel. On the drivers side that a piece of cake. The other side with having to remove that drop section is were I question the repair. This is only a 2,000 pound trailer so a piece of box tube, one piece with a correct angle for the hitch I believe will be more then strong enough. I was going to fish plate about 5 or 6 times in the length with 4” flat straps through bolted, no welding, for each side.