My 2001 V-10...50,000+ miles, have always used 10-30 fossil. (NAPA...only because there's a store nearby). No engine problems at all.
Your money, your choice.
My toad is 4000 pounds (2011 Ford Ranger 4WD) and I get 7.3 pulling it. I don't know about the slideout getting damaged while extended for a long period of time. The "seasonal" RV owners leave them out all "season" and I presume almost all retract them for the winter. (Spray the slideout guides AND the hydraulic jacks chrome pistons at least 3 times a year with silicone spray)
Oil additives, like STP are for 30-40 years ago. Today's motors simply do not need them...and the added thickness to the oil could cause a problem. IMO.
Enjoy your motorhome!