Not answering your question, but if things are not too bad, why replace them? Plumbing is not hard to do... use PEX rather than copper and you will be good for another 40 years. What is wrong with the electrical? The 12 volt
side generally holds up as well as the 120 volt side. Remove and replace the 120 volt outlets with new ones, and that is about all that needs doing there. If the 12 volt
side just needs updating, put a new converter/charger in and add new 12 volt
fixtures, fluorescent or LED's. Water pumps can be replaced inexpensively (under $100), a new furnace is about $600 and will have electronic ignition. Water heaters are in the range of $400 and also have electronic ignition, which is very nice (no pilot lights). Curtains can be replaced, curtain pull tabs are still available. New faucets freshen up the kitchen sink and bathroom sink. New shower head will make your shower enjoyable. New carpet or other flooring always brightens up an older trailer. New toilets are good, and relatively inexpensive. New Formica countertops are another thing to update an older unit. Painting the "old banana yellow" end caps with Krylon paint for plastics will restore the white they were originally. Same with painting the refrigerator vent plastic, and the AC cover.
In short, unless the interior cabinets themselves are falling apart, the urge to "take it all out" may not be the best idea, in my opinion. Too many people gut a trailer and then never get it back together, as that can be a daunting thing to do. It is especially bad when they throw away all the curved parts which fit the Airstream and are very hard to replicate later.
You can update and replace things one at a time as budget allows and not be without a camping unit.