@Chris (aka Eric Hysteric) -
Welcome, I hope you stay around ... it's encouraging to know this thread has trans-world participation.
I'm not sure what the "Tec" means in your motorhome description. A quick Google search seems to indicate a replacement engine, with a Magnum or diesel engine. In 1979, the 5.9L (360 cu. in.) engine was part of the LA ("Light A block") family of engines.
Alternatively, it may be a somewhat unique Dodge configuration, specifically made for European markets ... I'm curious so am digging deeper.
Replacing OEM rims with 16" or 17" is a very common first "upgrade" for most older motorhomes, due to relative availability of tires. Unfortunately, there are pitfalls.
Older trucks almost always "lug-centric" versus the more recent "hub-centric" rims. The difference is using mounting bolts and nuts (i.e., "lug nuts") to align the rims to the axle hubs. Many later trucks, such my daughter's 2004 Chevy 2400HD Silverado, uses a raised ring on the hub and center hole in the rim for alignment.
Most older truck also have "coined" lug/bolt holes. Alternating rim mounting holes are indented or raised (conically stamped) to assist centering the rim on the hub. The following picture shows coined lug holes. The top red arrow points to a raised conical stamping and the bottom to a indented conical stamping.
The black arrow point to a hole for a dual wheel alignment pin found on some older (mostly 40s, 50s and 60s) trucks.
BTW - the coining (indented and raised) aligns on dual wheel rims.
Being hub-centric, most newer truck rims aren't coined. The result is it's easy to mount these rims off-center on older trucks, such as your motorhome. The following picture shows late-model 17" Chevy Silverado hub-centric dual wheel rims. Note the absence of coining on the lug holes.
IIRC, older Ford and Dodge dual wheel rims are identical ... and older Chevy trucks used a different bolt hole pattern. (I believe Ford and Dodge rims were manufactured by the same plant ... and the only difference was the pentastar or oval stamped inside the rim.)
Assuming the 16" rims will clear your front brake calipers and steering linkage, what you want are 16" coined dual wheel rims with a 8 x 6.5" bolt hole pattern.
Enough for now ... more info, experiences, and "horror stories" upon request.
