The sender is usually part of the fuel outlet assembly at the top of the tank. The pump sucks fuel out of the tank like drinking through a straw, If the top isn't accessible you'll have to drop the tank to get at it.
You know where it is in the coach, so two problems remain: identifying the sender part and finding one.
If you don't know the sender part ID, what chassis is the Ultrastar built on? If its Freightliner/Oshkosh, you may be able to get part identification from Freightliner Custom Chassis. Ditto if its a Spartan. Otherwise, not much help available that i know of.
If you know the part brand/number, you can try finding it in salvage yards, truck or RV.