4X4 Firefighter

Hi Ludel I have a 2006 Dodge Ram 2500 4X4 5.9L. My truck has 3:73 gears and 35 Toyo M/T 2 " level kit. I drove to Macon Ga. from Tampa Fl.@ 65mph and got 28 Mpg up I75 in March of this year. I am lifting my truck 6" and putting 37" Toyo MT on. I was going to put 4:30 Nitro Gears in. I also got 25-26 MPG from here to NC in Dec. of 2014. My MPG around town is 16-18 city driving. Now all of this is empty truck bed. Now i do plan on towing a camper to Ga every now and then and any where else i go for vacation. I was wondering being that my RPM's are around 1600 to 1700 at the speed mentioned above would i receive equal results doing my lift and gear change. I used a RPM calculator and for the given size tire the rpm's for a 4:10 set up were 1829 rpm and for a 4:30 set up 1913 rpm. Now i compared that to the set up I have now using the Calc. and my rpms are 100 rpm higher with the 4:30 set up and lower with the 4:10 set up than i currently have. I think you are reporting that the 5.9L sweet spot for fuel maximizing is around 2000 rpm, because you want the higher rpm range when traveling at 70 mph + to avoid high boost pressures. Oh my calculations were done at 70 mph. If I go to a 4:56 set up the rpm's go over 2000. Can you help. with a bit of knowledge.