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No access from any other point or underneath?
I have had to do that sometimes when you get to to it. When they build these they never think anything will ever break! I have also made an access sometimes if I can do avoid taking the tub out.
It can be anywhere you need it to be to get to the area that has to be accessed. When they build these things, many times they forget that things break! Plumbing included! Most times it can be from the side or directly under the shower from a service bay. Walls in the bays can be taken down if needed. I don't think in this case it would be from under the chassis, but I haven't seen your trailer. Weigh that option against removing the tub. See which is easier and lest costly. Attach a photo or two to your post if you can as the more I can see yours, the better advice I can give and I can be more specific.