RVs with a Dodge chassis came with 3 options in 1978.
Class C: full "C" channel frame front to back. Most had little or no nose, and often had a fiberglass front panel. may or may not have had a hood. really depended on which motor home company the RV came from. Most were big block chassis.

Class B: An extended Dodge B van chassis had a boxed steel front subframe to back behind the doors where it merged with a rectangle shaped boxed sheet metal "frame". The under carriage of these were strictly dodge B van unibody chassis all the way back to the rear box that would have been under the rear facing doors on the B van, right behind the rear spring shackles. Then a tube frame, or possible C channel frame, or more box formed sheet metal to the back bumper (again depending on who the RV was purchased from. These could have come with any Mopar engine offered in 78, from a slant 6 (fortunately most were at least 318s) up to a 440. These B chassis RVs all had the B van sheet metal to at least the back edge of the front doors.

The other option was the Dodge Transvan. it too started life out as a Dodge B van, but I wasn't blessed with seeing many of these in real life. I have no idea what was done behind the front doors.

A few more things to understand with the RVs in general. The RV manufacturers bought the chassis in bunches at a time (like maybe 100 + chassis at the same time). The model year of the finished product was the year it was ready for a customer. The actual chassis under it may have been a couple years old at that time. A 78 model year RV could have had a 76 chassis under it, with everything one would expect to see in a 76 model year chassis.

I spent may a day under the B van RVs repairing the rear frames. On every one of them, regardless of the manufacturers, the frame for the extension behind the rear springs was welded to that B van rear torque box. That rear B van box is 18 gauge sheet metal with doubled pieces of 18 gauge at anyplace Dodge felt they needed reinforcing in the original van chassis. That box was formed with an inner section spot welded to the outer section, the only thing between those inner and outer sections was those extra reinforcing pieces Dodge felt they needed to support the van's rear facing doors. The RV frame extensions were welded only to the rear section of that torque box. Then they added the the floor and the upper structure for the RV on top of that frame extension.

For the most part, it worked out pretty well, until people started pulling trailers with a lot of tong weight on the back of the RVs. After a few years, that extra tong weight started separating and crushing that rear torque box and the trailer hitch on the RV's rear bumper got a lot closer to the ground. The entire rear section of the RV started drooping. The fix was to jack the RV up at the trailer hitch to lift the back end close to where it was suppose to be (usually that involved lifting the rear wheels off the ground. Then 1/8" steel plates were welded to the torque box and it was welded back together. Then add a 4' long piece of 2" square tubing under the B van rear "frame rails" and weld a 3' long piece of 2" square tubing to the RV frame extension (the RV extensions were usually 4' long), then bridge the repaired torque box with another 3' long piece of 2" square tubing welded overlapping both the front and rear tubing. Then I had to add a 2" high stand on the lower side of the bridge tube at the center, and add a 1/4" thick 2" wide flat steel bar to the ends of the tubing on the frame and across the 2" tall stand. If the customer reduced the tong weight on the trailer to around 600 lbs, that patch job held up several more years. if they didn't reduce the weight, or failed to keep the extra weight off the tong, the patch job was only good for a couple years. If the patch failed, there was no farther repair. I've probably done that patch job to 20 or more class B Dodge motor homes. Trust me when I tell you how those class B Dodge motorhomes with the van front sheet metal were built. Gene