I might have missed it but I did not see your trip destination or starting point. From your comments about Luling and Castroville, I suggest not going around San Antonio. Stay on IH-10 westbound and it goes around the south part of San Antonio's downtown area. Watch the signs and stay on that road with no turns. As you get to IH-35, the highway changes to US 90, which takes you straight out to Castroville. The trick is not following the signs for IH-10 Westbound because it actually turns and runs on IH-35 Northbound to downtown San Antonio, then turns northwest to Boerne and on to El Paso. As long as you do not hit San Antonio during rush hour (say 3:30 to 6:30 p.m.), the highway is not too crowded and should be easily driveable with a trailer.
The Buc-ee's in Luling usually has the cheapest gas and diesel in this area, but it is cheaper in San Antonio itself. I don't go further west often enough to know about anyplace in Castroville or Hondo.
If I am wrong about your trip, loop 1604 is the outer loop around San Antonio and is a complete loop. Parts of it on the south and east sides of town are still two lane country road. IH-410 is the middle loop around the main part of the city. It is interstate all the way around. There is an inner loop (sort of) formed by the way IH-10, IH-35, and IH-37 intersect and form a square around downtown (actually the legal definition of downtown in San Antonio is the area inside those highways). I recommend staying away from the inner part when possible.