Try to get as much of the old stuff off as you can then clean it real well with isopropyl alcohol. The Dicor will cover everything and self level. Good Stuff. The other option is Eternabond Tape, never tried it myself but heard it’s pretty good. – http://www.eternabond.com/RV-Leak-Repair-Products-s/22.htm
We started out new on both, we had a truck camper before on my 1500, was just not enough room so we upgraded to a 5er and really love it.We went new because we didnt want anybody elses problem that may have come with used,not that used is bad. We just like new. You look like you have a nice setup too. We looked into buying a Heartland it was a tossup and we liked the floor plan we got with the High Country.