Presuming your Expedition is reliable and roadworthy, DO NOT buy a motorhome if you're on a tight budget. And if your Expe is not in good mechanical shape, spend the money on that or a more reliable vehicle.
Tight budget? Best bang for the buck is a tent.
If you must have a RV, I would suggest either a popup like some said or a small hard side camper, for the reasons already mentioned.
No need for another big engine and all the maint that comes with a heavy duty vehicle and license, insurance, etc, if you already own one.
And if you don't think you can tow a small trailer (skills wise), you will be challenged in a moho as well.
Good Luck!