Cross member replacement

That is basically what we did on our Trail King posted above (similar to a Landoll). As long as you close the hole back up, it will be fine. Most of the trailer strength is in the top and bottom flanges, the web is just holding the flanges apart.

 

I did mine several years ago, the material wasn't all that expensive. It is a lot of labor though.

With the price of steel and everything else skyrocketing lately, and labor so high (unless you do it yourself), it's not going to be cheap.
I assume your Landoll is a older one with tubing crossmembers, that is how our Trail King was. Worst thing to use for crossmembers, they trap moisture inside and rust bad. When we replaced ours we went back with heavy 3" beams. We have since sold the trailer, but the beams held up fine, and I would imagine the rest of the trailer will be junk by the time it needs crossmember replacement again.