Dodge started using the designation RM300, RM350, and RM375 in 1973 though there are some 73's with the pre RM desgination. In the 74/75 suplement manual they then changed that back to M300, M400, and M500 in 75 and said it applied backwards to 1973 for RM designations. Made things confusing for many. Use the following translation
For 1969 - 1973 (with 318/413)
M300 or M375 as applicable
Note: there are early (69/71) and late (72-73) versions to add to the confusion
For 1973 - 1975 (with 318/440)
RM300 = M300
RM350 = M400
RM375 = M500
Note: To confuse things more you could see either designation in a given year.
1975 and later (with 318/360/440; 318/360 availability varied by year)
Get yourself a copy of the Dodge Service and Parts manuals. The least expensive place to get them in PDF form that I know of is the online store at CWVRV: Classic Winnebagos & Vintage RVs!
For a 75 chassis, you also need the 74/75 supplement.
The unfortunant part is that while the parts book shows all the parts up through 1977, the service manuals did not get updated to reflect the hydroboost system until sometime after 75. If you want detailed service information for the hydroboost system you have to get a later Service manual also. I know the 78-79 version has hydroboost in it. I am not sure about 76/77.
I have found the best approach for Dodge Class A MH chassis parts is to get the Dodge P/N first then use parts site cross reference data. Many parts sites do not list the MH chassis but you can cross the p/n (NAPA, Bendix, Raybestos, Autozone, Wagner, etc.). The parts sites have as much trouble trying to decipher what Dodge did for chassis designation as the common user does.
Depending on what you looking for, you can sometimes use the D300 (2WD) or W300 (4WD) Dodge pickup chassis designation for parts. Mostly Dodge generic stuff like ignition or charging. Be aware the 440-3 engine uses a special skirted thermostat that is basically only available from Mr. Gasket. The typical car 440-1 type thermostat will not route the water correctly on a 440-3. The 440-3 heads are different from 440-1.