You'll want something closer to 5k dry, maybe 6k. What year is your Expedition? They will be payload or receiver limited with a travel trailer. Towing an 8500 pound RV with a family of 5 will put an Expy well over some of its other ratings.
Pick your length based on what campsites you want to fit in. Some people travel in 45' motorhomes full time across the country. There are plenty of sites for them, just maybe not where
you want. Length is not necessarily relative to weight. There are 25' TTS that will weigh 5000 pounds loaded and 10k loaded.
RVs all use the same 2-3 brands of parts, appliances, water heaters, slides. I personally would not care if the brand is not longer made. For shopping used, the condition and how it was maintained is far more important than brand. Water intrusion is the single most costly issue. So look under/behind everything for evidence of leaks.
One benefit of buying used is the cost is low if you decide you don't like RVing, or the exact model/floorplan you like. Keeping it for one season will depreciate maybe $1000.
There are only so many ways to fit 3 beds, a kitchen, living, and bathroom into 200 square feet of living space. Find the one that works for you best. Role play as you shop. Do your knees hit the wall/door when you sit on the toilet? Can someone walk by when cooking? Can you kneel to bath your toddlers? Do your feet and head simultaneously touch walls lying on the bed? Is there space for a weeks worth of clothes, food, cookware, dishes? I recommend browsing an RV show where many types of floorplans are without too much hassle fro ma salesman. Once you find a floorplan, we can help you find a used model with that variation.
A & A parents of DD 2005, DS1 2007, DS2 2009
2011 Suburban 2500 6.0L 3.73 pulling 2011 Heartland North Trail 28BRS
2017 Subaru Outback 3.6R
2x 2023 Chevrolet Bolt EUV (Gray and Black Twins)