I can't speak for rear sag as I have air suspension, but I do think there's value in E load rating tires if you tow. The SL rated tires on my ORG truck are squishy which is fine driving unloaded but when towing I'd like more stability.
As for deciding between Terra and Ridge, my $0.02 is the tread pattern on the RG does not look good for snow and wet, but there are many on here with these tires that can chime in. I am considering the TGs for when I upgrade. Probably 275/65 or 295/60, I think 34" is perfect.
One unique thing that Nitto offers is the TG in 275/65R20 with E load or SL load rating. But again for towing Id stick with E.
BTW, nice camper! Grand Design is sweet.