How long until your son is 18?
You indicated that once he is an adult, you'll move away from the DC area. Will you still have the same income? Or will that potentially change?
If it's only 1-5 years left until your son is 18, why not just continue to rent for that time. While waiting, continue to build your credit. Look to get a second card and use both each month - paying them off in full each month. That will establish a history of on-time payments (which is the major factor in credit scores).
After the first year or two, find the tow vehicle you want and buy it. Finance it for 2-3 years and that will build up your credit even more.
When you pay off the tow vehicle, find the RV you want and either buy it outright (if you can), or finance part or all. By that time, you will have enough credit history to do so.
That puts you in a position to fulltime in the RV within 3-5 years. Your son will be an adult and you won't need to worry about the ex starting another custody fight. You can move out of the area, roam the country, move near your son's choice of college, or just reinvent yourself.