Separate bedroom for you and child means a bunk house of some kind. Only one child, so a double bunk will do. Spend some time looking at floorplans of RVs. There are many brands and variations of very similar floorplans. Then you can pick a weight and appropriate tow vehicle (TV).
4 miles to work each way is nothing. You will put a lot more miles towing an RV than daily driving, so the tradeoff of more capability vs size/fuel economy favors larger vehicles.
Towing any high walled RV is hard work due to the aerodynamic drag of the travel trailer. Gas engines tend to get 8-10mpg towing. Diesels maybe 12-15mpg. If can find something under about 4500 pounds dry, the 2014 Jeep Grand Cherokee Diesel would be good along with 2012+ Vw Touareg TDI.
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)