DIY. Your AS is almost vintage so it can be your hobby to maintain it and upgrade it from now on. Avoid making the dealer unnecesssarrily richer to do this easy one day max job for you. Start by exposing any rotted wood. Open up *only* the bare minimum metal to expose the extent. You should plan to approach from all approachable paths, e.g. top-from-inside-of-trailer, outside-from-removed-front-rail-panel, underneath via belly skin panel remove, etc. If rot is VERY extensive then you'll need to plan to remove & replace by adding suitable sturdy and durable material, like aluminum plating (from big-box store or any metal dealer; you can cut alum w/ a skil-saw metal blade, or similar; rout a "lip" on the good floor exactly deep enough to secure the alum plate FLUSH to the good floor use flat-head stainless-steel screws, or for added lifetime but longer job time use flat-head SS through bolts. (I don't think you should consider using any wood product to replace because the same problem will likely reoccur in 20 years time.) If rot is limited in its range, as I suspect, then it's also likely it will all be removable by a combo of wire-brush/chip/sand. Get it all clean and dry, then apply Bondo, yes Bondo. It is a tried and proven repair product. It's hard, durable, rot-proof, termite-proof, very shapeable, very water-resistant, and anyone who can already pull an AS Excella trailer can learn to work really well with Bondo in less than 20 minutes of practice and watching YouTube How-To videos - a gallon can of Bondo will probably be more than you need for this job and costs only about $35 to $40, at last look - it's also sold in qts & pts. Grind-to-shape/apply-finish coat(s)/sand smooth, then "button up" everything and get back to enjoying this beauty with the satisfaction that you did it yourself and saved a ton of money in the process.
Newbie poster but long time owner and always the DIY maintainer/upgrader of my one-family, 60 years young 26' 1956
Overlander-Cruiser whaletail, tandem axle, #6713, that still gets towed several thousand miles each and every year! (Yoda said, "When 900 years old you are, look as good you will not." That staement aptyly describes what having a vintage AS is ALL about