We are planning our time in Mexico for 2013/14. We've been down three times before with 5th wheel (Tenacatita; Lo de Marcos) but are changing it up this year and coming with our truck camper. Hopefu...
Since you have already done the coast 3 times, and this time coming down in a TC, how about doing the Colonial area of Central Mexico. With the TC, you can go anywhere, and stay in all the RV parks. While you're at it, also visit the cities surrounding Mexico City. Then, if you want, drop down to Oaxaca, and come back up the coast. I'm doing a similar trip this summer, but won't go to Oaxaca, as it is a long haul, and there are many more interesting cities to visit.